Remove usages of Commons Lang 2

[timja-bot]

No need to depend on a third-party library here when the Java Platform provides this
functionality natively.

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

• FolderAuthorizationStrategyAPI: the three StringUtils.isBlank(sid) checks become
  sid == null || sid.isBlank().
• FolderAuthorizationWebAPITest still needs StringUtils.capitalize, which has no JDK equivalent,
  so io.jenkins.plugins:commons-lang3-api is declared with <scope>test</scope> and the import
  moved to Lang 3 — src/main stays dependency-free.

Testing done

mvn -B -ntp clean verify passes locally on Java 21 / macOS.

The ban-commons-lang-2 enforcer rule was left disabled: it needs parent POM 6.2116.v7501b_67dc517 or newer. I tried the bump, but the newer SpotBugs
then fails on a pre-existing US_USELESS_SUPPRESSION_ON_METHOD for NP_BOOLEAN_RETURN_NULL in
AbstractAcl.hasPermission — an unrelated cleanup, so I left the parent alone.

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.