Rename GitHub Action Metadata File

[threeal]

This issue suggests renaming the GitHub action metadata file from action.yaml to action.yml. This change is proposed because apparently GitHub does not recognize if the project contains a GitHub action if the action metadata file is named as action.yaml instead of action.yml. This behavior can easily be evidenced by creating a new release where the option to publish the action to the marketplace won't be available if the action metadata file is named action.yaml.

The ichiro-its/ros2-ws-action action also uses the action.yml name as the metadata file name for the same reason. Back then, the option to publish the action to the marketplace was not available when creating a new release.