chore: add documentation about the migration from Acegi Security to Spring Security for improved security and compatibility

[gounthar]

Summary

• Documentation
  • Added a new section outlining updated security integration. This update emphasizes enhanced compatibility with modern Jenkins versions, a reduction in false security alerts, access to streamlined security features, and improved community support.

Testing done

mvn clean install

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 demonstrates feature works or fixes the issue