Release FF `environment_stop_actions_include_all_finished_deployments`

What does this MR do and why?

Allow users to cleanup partial resources from f... (#435128 - closed) was introduced behind the last_deployment_group_for_environment feature flag. The change introduced a new method Environment#last_finished_deployment_group, which is called in place of Environment#last_deployment_group in Environment#stop_actions.

This MR:

• Release FF environment_stop_actions_include_all_finished_deployments
• Remove all code relating to Environment#last_deployment_group
  • Environment#last_deployment_group
  • Environment#latest_successful_jobs
  • Deployment#last_deployment_group_for_environment

The feature flag has been enabled globally on Gitlab.com: #435132 (comment 1876055796)

MR acceptance checklist

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Screenshots or screen recordings

See !141216 (merged)

How to set up and validate locally

See !141216 (merged)

Related to #435132 (closed)