Resolve "Resource `gitlab_runner`'s `token` attribute should be marked as sensitive"

Closes #6090 (closed)

This MR marks the token output from the gitlab_user_runner as sensitive, since it is a secret. This may break users who are currently implementing this resource, which is unfortunate since it was just released. However, it would likely be a minor breaking change, given the resource was just released, and it would be considered a security update as it adds protection to a secret.

It's a small change to the codebase, so I went ahead and made it, and we can merge it later or close the MR if we don't want to make it in %16.4 .