Add setting to only allow merge requests to be merged when all discussions are resolved

Originally opened at !6385 (closed) by @rodolfoasantos.

What does this MR do?

Based on #20968 (closed) this merge request adds setting only to allow merge requests to be merged when all discussions are resolved.

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

Check if there are other points to add the resolved discussion setting

Why was this MR needed?

Add the possibility to configure the project to only accept merge request when all discussions are resolved

Screenshots

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• 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?

Closes #20968 (closed)