Accept minty.yml as well as minty.yaml

Currently we only look for `.github/minty.yaml` when looking for a v2 config.

https://github.com/abcxyz/github-token-minter/blob/6c5fb9321c0304db1023a95cf9c289f69f74b838/pkg/server/config.go#L81

Generally we use `.yml` for yaml extensions in our GitHub workflows, so I would expect it to be supported. We should search repos for files with both extensions.