Migrate from Commons Lang 2 to Commons Lang 3

[timja]

Migrate from deprecated/EOL Commons Lang 2 to Commons Lang 3.

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).

What's changed

• Renamed the org.apache.commons.lang.* imports to org.apache.commons.lang3.* and declared
  io.jenkins.plugins:commons-lang3-api (BooleanUtils.toBoolean, NumberUtils.toInt and
  StringUtils.trimToNull have no clean JDK equivalent).

Testing done

mvn -B -ntp clean verify fails in CompareCoverageActionTest on my machine — that test errors
identically on an unmodified checkout, so it is pre-existing and unrelated.

The ban-commons-lang-2 enforcer rule was left disabled: it needs parent POM 6.2116.v7501b_67dc517 or newer, and parent 6.x requires Jenkins 2.475+
(Jakarta EE 9) while this plugin targets 2.319.1.

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 @timja.