Deprecate returning "token" in Runners API

Because "token" attribute is no longer used in UI and it creates a security concern, we have decided to remove it from public detailed Runners API endpoints.

In this issue %12.10:

• We will deprecate this token with documentation change and deprecation notice in serializer.

Issue for %13.0:

• We will remove this attribute from both documentation and serializer in #214322 (closed).

There is a long discussion in #195842 (closed) (confidential issue).