Do you mean normalize rather than standardize? By far the most common extension for GitHub configuration is .yml and that is what's documented in: the example workflow tutorial, example action tutorial (the reference prominently declares action.yml as preferred upfront), the sponsor button how-to only mentions FUNDING.yml, the Dependabot reference only mentions dependabot.yml, the examples in the official starter template repo only use .yml, etc.

https://yaml.cc/tutorial/yml-vs-yaml.html the official spec recommends yaml.