[GHF][BE] Move merge rules to yaml (#84065)
To allow comments
Update `trymerge.yaml`, `revert.yaml` and `tryrebase.yaml` to use v4 setup-python action and install pyyaml
Reformat json to yaml by running:
```
python -c "import yaml;print(yaml.dump(yaml.safe_load(open('.github/merge_rules.yaml')), sort_keys=False))"
```
Pull Request resolved: https://github.com/pytorch/pytorch/pull/84065
Approved by: https://github.com/b0noI, https://github.com/huydhn
