Making required Jenkins version consistent between maven and docs.

[Tzrlk]

The minimum Jenkins version reported by the plugin page (on the upper left) does not match the one specified in README.md. The version of Jenkins tools referenced in the Maven pom also matches the version reported by the plugin page, indicating that the documentation is out-of-sync, and should be updated.

Testing done

No testing required.

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