Touches #33007.

As of #71806, per-replica circuit breakers are off by default. Once they are in
a state suitable for production use, the default should be flipped to "on", and
this must happen before the 22.1 release branch is cut.

This should be accommodated by additional testing and by a review of all issues
linked to #33007 to determine which one are required before the flip.

The commit that defaults breakers to on should also add the release note, which has intentionally been held back.

Jira issue: CRDB-12220