[JENKINS-73316] Require Java 17 and 2.475; adapt test suite to EE 9

[basil]

See JENKINS-73316. Allow this plugin's test suite to be exercised regularly in Plugin Compatibility Tester (PCT) in https://github.com/jenkinsci/bom by requiring Java 17 or newer and Jenkins 2.475 (released today) or newer.

This PR goes against the usual advice from https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/ by requiring a weekly release. The advantage is that this will allow plugin compatibility testing (PCT). The disadvantage is that new features and bug fixes can't be released to LTS users for another few months.

The choice is up to the maintainer as to whether or not to accept this PR. If this PR is rejected, we will stop testing this plugin in PCT.

[basil]

The update to Jetty 12 (EE 9) in Jenkins core (hopefully for the next LTS) will break org.jenkinsci.plugins.saml.OpenSamlWrapperTest, which we test in BOM.

As described in https://issues.jenkins.io/browse/JENKINS-73316 one solution to this problem is what is demonstrated in this PR. This fixes the test failure at the cost of requiring a weekly release with Jetty 12 (EE 9), currently scheduled for later in September or October. That goes against the advice in https://www.jenkins.io/doc/developer/plugin-development/choosing-jenkins-baseline/ to "Prefer an LTS version over weekly versions", but it is not unheard of in Jenkins plugins to depend on a weekly release, especially where there are complex breaking changes in core involved (of which this is surely an example).

@kuisathaverat Are you OK with the approach taken in this PR (depending on a weekly release of core)? If not, are you willing to try the alternative approach described in https://issues.jenkins.io/browse/JENKINS-73316 to remove Mockito usages from OpenSamlWrapperTest? If neither option is feasible, then we might have to disable this test in BOM/PCT (which is also undesirable because it reduces test coverage against recent weekly releases).

[kuisathaverat]

It is fine, I will need the Java 17 requirement to bump the pac4j dependency

[kuisathaverat]

released as https://github.com/jenkinsci/saml-plugin/releases/tag/4.485.v99810fb_34d77

[basil]

Thank you very much!