Deprecate "token" attribute from Runners API

Merged Furkan Ayhan requested to merge 214320-deprecate-returning-token-from-runners-api into master

What does this MR do?

More info is in description of #214320 (closed).


Does this MR meet the acceptance criteria?


Availability and Testing


If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

  • Label as security and @ mention @gitlab-com/gl-security/appsec
  • The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
  • Security reports checked/validated by a reviewer from the AppSec team
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