Standardise rotation_configuration for group service account tokens

GitLab have added the ability to set the expires_at date in the group service account API in 17.9: gitlab#505671 (closed)

This means we can amend resource_group_service_account_access_token to have expiration_days in the rotation_configuration like all other types of token. This should also simplify the rotation logic and fix some bugs that occur due to the plan data being modified in the provider.