Disallow runner registration if allow_runner_registration_token disabled

What does this MR do and why?

Describe in detail what your merge request does and why.

This MR updates the Ci::Runners::RegisterRunnerService class to take the instance and namespace allow_runner_registration_token settings into account. This is described in the architecture blueprint https://docs.gitlab.com/ee/architecture/blueprints/runner_tokens/#details:

Part of #404804 (closed)

Screenshots or screen recordings

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How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Disable setting in Rails console:

   group = Group::last
   group.update!(allow_runner_registration_token: false)

2. Try to register a runner using the registration token (can be retrieved using group.runners_token). You should receive an error:

   gitlab-runner register  --url http://gdk.test:3000 --executor shell --non-interactive --registration-token "$REGISTRATION_TOKEN"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.