Standardize branch name params as `branch` on V4 API

This MR enforces the pattern of using branch instead branch_name on our V4 API params.

Changed endpoints:

• POST :id/repository/branches
• POST :id/repository/commits
• :id/repository/files CRUD endpoints

---

• 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?

Closes #22132 (closed)

EE-backport: gitlab-ee!1276