[FF removal] Remove group_managed_accounts FF and the accompanying code

Summary

This issue is to cleanup the group_managed_accounts feature flag, along wth the accompanying code. See parent epics for the various aspects of Group managed accounts code removal

Creates new way of encrypting tokens.

This is phase two, we roll out writer methods behind feature flag to avoid the same pitfalls as in first deploy.

Owners

• Team: ~"group::authentication and authorization"
• Most appropriate slack channel to reach out to: #g_manage_auth
• Best individual to reach out to: @ifarkas
• PM: @hsutor

Expectations

What might happen if this goes wrong?

Cleaning up the feature flag

• Create a merge request to remove <feature-flag-name> feature flag. Ask for review and merge it.
  • Remove all references to the feature flag from the codebase.
  • Remove the YAML definitions for the feature from the repository.
  • Create a changelog entry.
• Ensure that the cleanup MR has been deployed to both production and canary. If the merge request was deployed before the code cutoff, the feature can be officially announced in a release blog post.
  • /chatops run auto_deploy status <merge-commit-of-cleanup-mr>
• Close the feature issue to indicate the feature will be released in the current milestone.
• If not already done, clean up the feature flag from all environments by running these chatops command in #production channel:
  • /chatops run feature delete <feature-flag-name> --dev
  • /chatops run feature delete <feature-flag-name> --staging
  • /chatops run feature delete <feature-flag-name>
• Close this rollout issue.