Remove Ci::Runner resource type handling from Authz::CustomAbility

What does this MR do and why?

1. Update RunnerPolicy to check for if user has the read_runners custom ability on the runner's owner Project or Group
2. Remove handling of Ci::Runner from Authz::CustomAbility – custom abilities should always be checked against a project or a group they are granted for

References

https://gitlab.com/gitlab-org/gitlab/-/issues/576900+.

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