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

• ReverseProxySecurityRealm: the seven StringUtils.isBlank / isNotBlank calls become null
  checks plus String.isBlank(). (ReverseProxyAuthenticationProvider uses Spring's StringUtils,
  which is untouched.)
• Bumped the parent POM to 6.2211.v27f680c93c53 so the ban-commons-lang-2 enforcer rule is
  available, and enabled it. The reformatting in that file is mvn spotless:apply output.

No dependency was added — this plugin did not declare Commons Lang, it came in transitively from
core.

Testing done

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

The ban-commons-lang-2 enforcer rule is enabled in this PR, so the build fails if an
org.apache.commons.lang.* import comes back.

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.