Update php.yml to remove unnecessary GITHUB_TOKEN permissions

[matsuo]

References:
https://docs.github.com/en/actions/tutorials/authenticate-with-github_token#using-the-github_token-in-a-workflow

As a good security practice, you should always make sure that actions only have the minimum access they require by limiting the permissions granted to the GITHUB_TOKEN.

https://docs.github.com/en/actions/reference/workflows-and-actions/workflow-syntax#permissions

Work with the contents of the repository. For example, contents: read permits an action to list the commits