Change target branch in compare form automatically

What does this MR do and why?

Created an internal API that will get called whenever the source branch dropdown value. This fetches and returns the target branch for the specified source branch.

Screenshots or screen recordings

Screen_Recording_2023-09-06_at_16.15.22

How to set up and validate locally

• Enable target_branch_rules_flag feature flag
• Visit a projects merge requests settings
• Create a new target branch rule in the form at the bottom of the page
• Create a new branch with the branch name from the rule you just created
• Click new merge request button on merge requests list page
• Select the branch you just created
• The target branch should change to match the rule

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

#17909 (closed)