Migrate from Commons Lang to native Java Platform functionality

[timja]

No need to use a third-party library when this functionality is available in the Java Platform.

Part of the effort to remove Commons Lang 2 from Jenkins core — jenkinsci/jenkins#16404,
jenkinsci/jenkins#26105. Commons Lang 2 is EOL and carries an unfixed advisory
(GHSA-j288-q9x7-2f5v).

• StringUtils.equals(a, b) → Objects.equals(a, b) (three call sites).
• StringUtils.isNotEmpty(x) → Util.fixEmpty(x) != null.
• StringUtils.join(ignoreProjects, '\n') → String.join("\n", ignoreProjects), guarded on null.
  ignoreProjects is annotated @CheckForNull and StringUtils.join(null, ...) returns null, whereas
  String.join would throw — so getIgnoreProjects() keeps returning null rather than starting to
  throw for a project that has never had the field set.

Testing done

mvn -B -ntp clean verify passes locally on Java 21 / macOS.
The ban-commons-lang-2 enforcer rule was left disabled because it needs parent POM
6.2116.v7501b_67dc517 or newer; bumping the parent from 5.17 was out of scope here.

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests that demonstrate the feature works or the issue is fixed

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🤖 This pull request was generated with AI assistance (Claude Code) as part of a bulk migration
across Jenkins plugins. If anything here looks wrong, please comment on this PR or contact
@parameter.