Remove `:release_issue_summary` feature flag

What

Remove the :release_issue_summary feature flag. This feature flag is will be enabled by default.

We should remove this flag once the "release issue summary" feature (#31289 (closed)) has been live for a while without any issues.

Owners

• Team: ~"devops::release"
• Most appropriate slack channel to reach out to: #s_release
• Best individual to reach out to: @nfriend

Roll Out Steps

• 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