Use symfony/validator for as much validation logic as we can throw at it.

[GuySartorelli]

In silverstripe/silverstripe-framework#11123 we added a lightweight wrapper around symfony/validator, which has a ton of validation constraints we can use all over the place if we want to.

Some obvious quick wins:

• validation of URLs anywhere we're doing that
• validation of email addresses from forms (i.e. EmailField)
• validation of email addresses elsewhere (or else use egulias/email-validator validation for everything email related)
• password strength validation (which checks entropy instead of arbitrary rules)
• file and image validation (we probably do a lot of custom checks that this can do with less code on our end)
• locale and language codes (I don't think we currently check those for i18n stuff, but we should)

Because this would represent a change in logic, it could break some things so it needs to either be opt-in where it's applied or just be done in a major release.

Acceptance criteria

• Compile a list of things where existing logic could be replaced e.g. long regexs (ended up just using the list of constraints as my list of things to check)
• Use symfony constraints for the easy stuff
• [ ] Raise new cards for anything that is harder no need
• For password strength validation (and possibly others, though probably not) - it's still possible to implement their own business logic i.e. disable symfony constraints

Validation applied:

• URLs
• Email addresses
• Ip addresses
• Passwords

File and image validation would require changing too much code surrounding the validation itself, and locale stuff didn't seem worthwhile.
Most other validation was for things like "not blank" which is trivial to do without relying on symfony (or in the case of RequiredFields our setup is currently too idiosyncratic to rely on something that simple)

CMS 5 PRs

• DEP Deprecate API that will be removed or renamed silverstripe-framework#11401
• API Deprecate isValidEmail() method silverstripe-contentreview#262
• DOC Document some more deprecations developer-docs#589

CMS 6 PRs

• ENH Use symfony/validation logic silverstripe-framework#11399
• ENH Use symfony/validation logic silverstripe-linkfield#338
• ENH Use symfony/validation logic silverstripe-realme#167
• ENH Use symfony/validation logic silverstripe-graphql#609
• ENH Use symfony/validation logic silverstripe-contentreview#261
• ENH Use symfony/validation logic silverstripe-cms#3009
• DOC Document using symfony/validator logic developer-docs#590