Refactor MergeRequest::BuildService

What does this MR do?

Refactor MergeRequest::BuildService:

• add attribute merge_request to class to reduce parameters list of instance methods
• add delegations to merge request to remove duplicated merge_request. getters
• replace conditionals in validate_branches with add_error method
• refactor execute method

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

Why was this MR needed?

See #24795 (closed)

Screenshots (if relevant)

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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Fixes #24795 (closed)

/cc @adamniedzielski