maintainers/scripts/update.nix: Remove unicode from message and comply with CONTRIBUTING.md

It's not used infrequently and the ascii version is nicer for automatic parsing.
```
git log --grep '->' --oneline --since '2022-01-01' | wc -l
67791
git log --grep '→' --oneline --since '2022-01-01' | wc -l
2914
```

maintainers/scripts/update.py

···
         # Git can only handle a single index operation at a time
         async with merge_lock:
             await check_subprocess('git', 'add', *change['files'], cwd=worktree)
-            commit_message = '{attrPath}: {oldVersion} → {newVersion}'.format(**change)
+            commit_message = '{attrPath}: {oldVersion} -> {newVersion}'.format(**change)
             if 'commitMessage' in change:
                 commit_message = change['commitMessage']
             elif 'commitBody' in change: