Revert branch field removal in UserSquash RPC
This reverts a chunk from !1786 (merged) and instead marks the field as deprecated.
Older versions of Gitaly continue to require the branch field in order to pass RPC request validation. To prevent issues during upgrades, the branch field should be retained until the next release (12.9 or later).
See gitlab!23902 (comment 278419683) for more information.
Edited by GitLab Release Tools Bot