build: Test on the head of the release branches. (#38596)

Purposefully not adding the older releases here but for any new releases
we should test on the branch once PRs are merged to make sure there are
no regressions that were missed by weird merge timing.

Author: feanil

.github/workflows/unit-tests.yml

@@ -8,6 +8,7 @@ on:
   push:
     branches:
       - master
+      - release/*
 
 concurrency:
   # We only need to be running tests for the latest commit on each PR