Force update the target branch after the target project changed in MR

What does this MR do?

Force update the target branch after the target project changed when creating a new Merge Request.

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

Compare class in JS may need refactoring.

Why was this MR needed?

Unchanging he target branch even after the target project changed when creating a new Merge Request is convenient for projects that has different default branches than master.

Screenshots (if relevant)

target-branch-selection-in-mr-from-forked-project-gitlab

Does this MR meet the acceptance criteria?

• Changelog entry added
• [n/a] Documentation created/updated
• [n/a] 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #27020 (moved)