Limit PHP support for CMS 6 to PHP 8.3 only

[GuySartorelli]

As per our major release policy:

At launch, a Silverstripe CMS major release supports all PHP versions in full support. PHP versions in limited support are not supported at launch by new Silverstripe CMS major releases.

According to the PHP supported versions page, PHP 8.1 already lost active support, and PHP 8.2 will lose active support on 31 December 2024 which is before CMS 6 will launch.

Therefore, of the versions of PHP currently stable, only PHP 8.3 is allowed at release (PHP 8.4 will release before then, but that will be handled under a separate card)

Acceptance criteria

• Docs indicate CMS 6 supports PHP 8.3 only at launch
• CI doesn't try to run anything with PHP 8.1 or 8.2 for CMS 6

PRs

• ENH Update PHP versions for CMS 6 supported-modules#40
• FIX Use PHP 8.3 compatible version of vfsstream gha-ci#148

The above must be merged first - some of the CI runs depend on it before they'll go green (notably config - I think the rest probably don't need it, from looking at them, but not 100% sure)

• DEP Limit PHP support for CMS 6 recipe-kitchen-sink#70
• DEP Limit PHP support for CMS 6 silverstripe-contentreview#250
• DEP Limit PHP support for CMS 6 silverstripe-testsession#92
• DEP Limit PHP support for CMS 6 silverstripe-staticpublishqueue#199
• DEP Limit PHP support for CMS 6 silverstripe-behat-extension#280
• DEP Limit PHP support for CMS 6 silverstripe-admin#1811
• DEP Limit PHP support for CMS 6 recipe-core#99
• DEP Limit PHP support for CMS 6 silverstripe-iframe#96
• DEP Limit PHP support for CMS 6 silverstripe-sharedraftcontent#252
• DEP Limit PHP support for CMS 6 silverstripe-registry#104
• DEP Limit PHP support for CMS 6 silverstripe-subsites#592
• DEP Limit PHP support for CMS 6 silverstripe-asset-admin#1486
• DEP Limit PHP support for CMS 6 silverstripe-spamprotection#125
• DEP Limit PHP support for CMS 6 recipe-blog#57
• DEP Limit PHP support for CMS 6 silverstripe-session-manager#211
• DEP Limit PHP support for CMS 6 recipe-content-blocks#46
• DEP Limit PHP support for CMS 6 silverstripe-securityreport#86
• DEP Limit PHP support for CMS 6 silverstripe-mimevalidator#82
• DEP Limit PHP support for CMS 6 silverstripe-restfulserver#132
• DEP Limit PHP support for CMS 6 recipe-cms#90
• DEP Limit PHP support for CMS 6 silverstripe-login-forms#196
• DEP Limit PHP support for CMS 6 silverstripe-externallinks#140
• DEP Limit PHP support for CMS 6 silverstripe-realme#155
• DEP Limit PHP support for CMS 6 silverstripe-versionfeed#118
• DEP Limit PHP support for CMS 6 recipe-form-building#39
• DEP Limit PHP support for CMS 6 silverstripe-campaign-admin#322
• DEP Limit PHP support for CMS 6 silverstripe-webauthn-authenticator#194
• DEP Limit PHP support for CMS 6 silverstripe-elemental-fileblock#59
• DEP Limit PHP support for CMS 6 silverstripe-config#121
• DEP Limit PHP support for CMS 6 silverstripe-linkfield#320
• DEP Limit PHP support for CMS 6 recipe-testing#25
• DEP Limit PHP support for CMS 6 recipe-services#39
• DEP Limit PHP support for CMS 6 silverstripe-siteconfig#174
• DEP Limit PHP support for CMS 6 recipe-reporting-tools#48
• DEP Limit PHP support for CMS 6 silverstripe-gridfieldqueuedexport#119
• DEP Limit PHP support for CMS 6 recipe-authoring-tools#40
• DEP Limit PHP support for CMS 6 silverstripe-framework#11345
• DEP Limit PHP support for CMS 6 silverstripe-segment-field#121
• DEP Limit PHP support for CMS 6 silverstripe-crontask#91
• DEP Limit PHP support for CMS 6 silverstripe-documentconverter#93
• DEP Limit PHP support for CMS 6 silverstripe-versioned#467
• DEP Limit PHP support for CMS 6 silverstripe-installer#377
• DEP Limit PHP support for CMS 6 silverstripe-textextraction#100
• DEP Limit PHP support for CMS 6 silverstripe-elemental-bannerblock#159
• DEP Limit PHP support for CMS 6 silverstripe-event-dispatcher#27
• DEP Limit PHP support for CMS 6 silverstripe-auditor#89
• DEP Limit PHP support for CMS 6 silverstripe-reports#192
• DEP Limit PHP support for CMS 6 silverstripe-frameworktest#200
• DEP Limit PHP support for CMS 6 silverstripe-environmentcheck#110
• DEP Limit PHP support for CMS 6 silverstripe-userforms#1318
• DEP Limit PHP support for CMS 6 silverstripe-sitewidecontent-report#102
• DEP Limit PHP support for CMS 6 silverstripe-graphql#598
• DEP Limit PHP support for CMS 6 silverstripe-lumberjack#166
• DEP Limit PHP support for CMS 6 silverstripe-ldap#87
• DEP Limit PHP support for CMS 6 silverstripe-taxonomy#121
• DEP Limit PHP support for CMS 6 silverstripe-hybridsessions#111
• DEP Limit PHP support for CMS 6 silverstripe-totp-authenticator#170
• DEP Limit PHP support for CMS 6 silverstripe-versioned-admin#358
• DEP Limit PHP support for CMS 6 silverstripe-assets#631
• DEP Limit PHP support for CMS 6 silverstripe-mfa#560
• DEP Limit PHP support for CMS 6 silverstripe-tagfield#302
• DEP Limit PHP support for CMS 6 dnadesign/silverstripe-elemental-userforms#99
• DEP Limit PHP support for CMS 6 silverstripe-elemental#1238
• DEP Limit PHP support for CMS 6 silverstripe-cms#2990
• DEP Limit PHP support for CMS 6 silverstripe-blog#777
• DEP Limit PHP support for CMS 6 recipe-collaboration#36
• DEP Limit PHP support for CMS 6 silverstripe-errorpage#119
• DEP Limit PHP support for CMS 6 bringyourownideas/silverstripe-maintenance#237
• DEP Limit PHP support for CMS 6 bringyourownideas/silverstripe-composer-update-checker#94
• DEP Limit PHP support for CMS 6 symbiote/silverstripe-advancedworkflow#549
• DEP Limit PHP support for CMS 6 symbiote/silverstripe-multivaluefield#116
• DEP Limit PHP support for CMS 6 symbiote/silverstripe-gridfieldextensions#414
• DEP Limit PHP support for CMS 6 symbiote/silverstripe-queuedjobs#443
• DEP Limit PHP support for CMS 6 tractorcow-farm/silverstripe-fluent#874
• DOC Limit PHP support for CMS 6 developer-docs#560