Only allow admins to rename default/protected branches (#33276)

Currently, anyone with write permissions to a repo are able to rename
default or protected branches.

This change follows
[GitHub's](https://docs.github.com/en/repositories/configuring-branches-and-merges-in-your-repository/managing-branches-in-your-repository/renaming-a-branch)
design by only allowing repo/site admins to change these branches.
However, it also follows are current design for protected branches and
only allows admins to modify branch names == branch protection rule
names. Glob-based rules cannot be renamed by anyone (as was already the
case, but we now catch `ErrBranchIsProtected` which we previously did
not catch, throwing a 500).

models/fixtures/access.yml

@@ -171,3 +171,9 @@
   user_id: 40
   repo_id: 61
   mode: 4
+
+-
+  id: 30
+  user_id: 40
+  repo_id: 1
+  mode: 2