Do not pass 'gitlab_realm' into CC 'AccessService#access_token'

What does this MR do and why?

After removing proxy-through-com logic for CS, we no longer need to pass 'gitlab_realm' to self-issue a token with the correct claim. Dropping this argument in AccessService#access_token invocations simplifies the code.

Also, we should not define GITLAB_REALM_ constants in the SelfIssuedToken class as these constants are used in the broader scope.
This MR extracts the realm constants into the Gitlab::CloudConnector module.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

How to set up and validate locally

The change is a no-op, as it is only a refactor.
Running Code Suggestions locally (as a smoke test) is one approach to check the impact (should see no difference).

Related to #439343 (closed)