Remove `graphql_individual_release_page` feature flag

What

Remove the graphql_individual_release_page feature flag once the GraphQL version of the Release page has been live in production for at least a milestone with no issues.

Owners

• Team: ~"group::release management"
• Most appropriate slack channel to reach out to: #g_release-management
• Best individual to reach out to: @nfriend

Expectations

What are we expecting to happen?

The Release and Edit Release page will fetch data from GraphQL instead of the REST API. There will be no user-facing changes.

What might happen if this goes wrong?

The individual Release page and/or the Edit Release page may fail to load.

What can we monitor to detect problems with this?

None that I know of 😕

Roll Out Steps

• Enable on staging (/chatops run feature set feature_name true --staging)
• Test on staging
• Ensure that documentation has been updated
• Enable on GitLab.com for individual groups/projects listed above and verify behaviour (/chatops run feature set --project=gitlab-org/gitlab feature_name true)
• Coordinate a time to enable the flag with #production and #g_delivery on slack.
• Announce on the issue an estimated time this will be enabled on GitLab.com
• Enable on GitLab.com by running chatops command in #production (/chatops run feature set feature_name true)
• Cross post chatops Slack command to #support_gitlab-com (more guidance when this is necessary in the dev docs) and in your team channel
• Announce on the issue that the flag has been enabled
• Remove feature flag and add changelog entry
• After the flag removal is deployed, clean up the feature flag by running chatops command in #production channel