[Feature flag] Cleanup `environment_details_vue`

Summary

This issue is to cleanup the environment_details_vue feature flag, after the feature flag has been enabled by default for an appropriate amount of time in production.

Owners

• Team: groupenvironments
• Most appropriate slack channel to reach out to: #g_environments
• Best individual to reach out to: @afontaine
• PM: @nagyv-gitlab

Stakeholders

Expectations

What might happen if this goes wrong?

Cleaning up the feature flag

• Create a merge request to remove environment_details_vue 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 environment_details_vue --dev
  • /chatops run feature delete environment_details_vue --staging
  • /chatops run feature delete environment_details_vue
• Close this rollout issue.

Flag location clean-up:

spec/features/projects/environments/environment_spec.rb:13:    stub_feature_flags(environment_details_vue: false)
spec/features/projects/environments/environment_spec.rb:28:      stub_feature_flags(environment_details_vue: true)
ee/spec/features/projects/environments/environment_spec.rb:14:    stub_feature_flags(environment_details_vue: false)
config/feature_flags/development/environment_details_vue.yml:2:name: environment_details_vue
app/controllers/projects/environments_controller.rb:12:    push_frontend_feature_flag(:environment_details_vue, @project)
app/views/projects/environments/show.html.haml:11:  - if Feature.enabled?(:environment_details_vue, @project)
app/assets/javascripts/environments/mount_show.js:62:  if (!gon.features.environmentDetailsVue) {