chore: close the analyzer log, widen the tests, drop dead build config

[NicklasWallgren]

• Close the file LogAndFileAppender writes to once the decorated mojo is done, rather than leaving one handle open per analyzed module. The listener reads the log back from the mojo, so modules building in parallel cannot close each other's file.
• Name the deepest cause and the captured compiler output in the message of a failed compiler based step. A forked mojo buries the reason several wrappers down, which made a class version mismatch surface as a bare stack trace.
• Add unit tests for ReactorCompletionTracker, including a concurrent claim, ViolationsFilterService, ExceptionUtils and the permissive and severity filtering of StepResults. 13 tests to 32.
• Add a Checker Framework integration test. Its fixture is built outside this project's target directory, the step skips every file whose path contains /target/.
• Remove the plexus-component-metadata execution, it generates nothing for a JSR-330 project.
• Document the Java 21 requirement.