Standardize branch name params as `branch` on V4 API (!8936) · Merge requests · GitLab.org / GitLab FOSS · GitLab

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