Add issue stats to release blocks on the Releases page

What does this MR do?

Adds issue stats to the release blocks on the Releases page as described in #31289 (closed). This is the other half of !19451 (merged).

This branch is currently rebased on this MR until it is merged: !18579 (closed).

Feature flag

The changes in the MR are hidden behind the release_issue_summary feature flag (the same flag used by the other half of this feature, !19451 (merged)).

However, there is one small change that is not hidden behind the flag. For technical reasons, it was complicated to hide a small styling change to the release header that was made as part of this MR. See the With the feature flag disabled screenshot below for a visual demo.

Screenshots

No associated milestones (no issue summary is shown)

One milestone

Two milestones

Lots of milestones

Video demo

https://youtu.be/bx2ecD359hc

With the feature flag disabled