docs: need admin scope for the runner tags test

For the MGCI_TEST_RUNNERS_TAGS test to work, we need to get runners tags info. The thing is, this information is only available to an access token with admin scope. Therefore, it can't be a project access token.

Edit: I did some testing and things are a bit trickier than this

• Project access token with scopes api, create_runner, manage_runner : KO
• Personal access token with scopes api, create_runner, manage_runner : OK
• Personal access token with scopes api, admin, create_runner, manage_runner : OK
• Group access token with scopes api, create_runner, manage_runner : KO

The personal access token must be from an instance administrator as stated here