Resolve "Clean outdated flaky specs from the flaky specs report"

What does this MR do?

This prunes flaky reports older than 3 months because that probably means that they've been resolved since then.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

Closes #37721 (closed)