Remove rbac_clusters feature flag

The rbac_clusters feature flag was introduced in !21127 (merged) and should be removed once https://gitlab.com/gitlab-org/gitlab-ce/issues/44597 is complete. (or https://gitlab.com/gitlab-org/gitlab-ce/issues/51942 is complete)

See also https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21127#note_98722913

Proposal

• Remove rbac_clusters feature flag
• Default to RBAC clusters enabled, both on the "create new cluster" and "add existing cluster" case
• Adjust QA to match, possibly remove --enable-legacy-authorization from Kubernetes tests. https://gitlab.com/gitlab-org/gitlab-ce/-/jobs/103932302
• Update documentation

Solution

• Remove rbac_clusters feature flag
• Leave RBAC cluster disabled as security model for RBAC is not ideal
• Update documentation (remove bit about toggling Feature Flag)
• Add QA for adding/creating RBAC clusters - https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/22025

Manual QA

Staging

• Enable RBAC
• Check "Create new cluster" scenario
  • Create cluster
  • Install all the applications
• Check "Add an existing RBAC cluster" scenario
  • Create cluster
  • Install all the applications

Production

• Wait for an 11.4 RC to land on production
• Enable RBAC
• Check "Create new cluster" scenario
  • Create cluster
  • Install all the applications
• Check "Add an existing RBAC cluster" scenario
  • Create cluster
  • Install all the applications