Fix sorting issues by "last updated" after import from GitHub

What does this MR do?

Don't touch Issue/Merge Request when importing GitHub comments as it will trigger an update on updated_at field. It also use updated_at as the last updated date doesn't matter the Issue/Pull Request state.

Why was this MR needed?

After import from GitHub, sorting issues by "last updated" doesn't work as expected.

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #21567 (closed)