CI (Buildbot, GHA): Simplify the permissions key in the workflow file for the "Statuses" workflow

[DilumAluthge]

According to the GitHub documentation (https://docs.github.com/en/actions/reference/authentication-in-a-workflow#modifying-the-permissions-for-the-github_token):

You can use the permissions key in your workflow file to modify permissions for the GITHUB_TOKEN for an entire workflow or for individual jobs. This allows you to configure the minimum required permissions for a workflow or job. When the permissions key is used, all unspecified permissions are set to no access, with the exception of the metadata scope, which always gets read access.

Therefore, if we just set statuses: write, all of the other permissions will automatically be set to no access.