Backport to v5: Add Maven compiler problem matcher for javac diagnostics

[brunoborges]

Description:

Backport of #1086 to the releases/v5 line.

The javac problem matcher registered by setup-java only recognizes javac's native diagnostic format (File.java:12: warning: msg). That works for plain javac and for Gradle, but not for Maven. The maven-compiler-plugin reformats every diagnostic to [WARNING] /path/File.java:[12,5] msg, so the existing regex never matches and Maven builds produce zero GitHub annotations even though the warnings/errors appear in the log.

This cherry-picks the fix, adding a new maven-javac problem matcher owner to .github/java.json that recognizes the Maven format and captures severity, file, line, column, and message. A separate owner is used because multiple patterns under one owner are treated as a multiline sequence, not as alternatives. Both owners are registered by the same existing ##[add-matcher] call, so no source change is needed.

Notes:

• The existing java and javac owners are unchanged, so current behavior is preserved.
• java.json is loaded at runtime by the runner and is not part of the webpack bundle, so dist/ does not need regeneration.
• GitHub matches the severity value case-insensitively, so capturing the uppercase WARNING/ERROR labels works.

Related issue:

Relates to #1085 (fixed on main by #1086)

Check list:

• Ran npm run check locally (format, lint, build, test) and all checks pass.
• Mark if documentation changes are required.
• Mark if tests were added or updated to cover the changes.