Enforce get method for validation checks

[miglen]

The following change enforces the use of get in the form validation instead of post which was recently introduced by default and it's breaking the plugin functionality.

• [ X] Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• [ X] Ensure that the pull request title represents the desired changelog entry
• [ X] Please describe what you did
• [ X] Link to relevant issues in GitHub or Jira
• [ X] Link to relevant pull requests, esp. upstream and downstream changes
• [ X] Ensure you have provided tests - that demonstrates feature works or fixes the issue

Related issue: https://issues.jenkins.io/browse/JENKINS-65790

[miglen]

Ping @hsbt @mitto @gogotanaka @ryanschwartz