Improve EECompilationParticipant.validateCompliance() to consider --release option

The issue is described here: https://github.com/eclipse-jdt/eclipse.jdt.core/issues/5281#issuecomment-5456254785

Copy-pasting what @stephan-herrmann said there:
```

I suggest we also improve org.eclipse.jdt.internal.launching.EECompilationParticipant.validateCompliance(String, IJavaProject, IVMInstall):

When --release is not specified, a mismatch is reported as
"The compiler compliance specified is X but a JRE Y is used"
When that option is specified, the relevant check is skipped, see
// validate compliance only if release flag is not enabled

I guess for fairness we should not silently drop a configuration option without giving some hint to the user.
```

Start with org.eclipse.jdt.internal.launching.EECompilationParticipant.validateCompliance(String, IJavaProject, IVMInstall) and read the linked issue comment for the expected behavior. Trace how compliance mismatches are reported with and without --release, then verify that the relevant user-facing hint is present without incorrectly applying the JRE mismatch check.

Written by the indexing model from the issue text.