Do not run CI on merges to relese (#19004)

PRs are merged to the release branches using our own implementation of
the merge queue. The history for them is linearised, and we are running
all the CI after every backported batch. Those builds are redundant and
can clog our runners.

[skip ci]

Author: Kordyjan

.github/workflows/ci.yaml

@@ -18,6 +18,7 @@ on:
       - '*'
     branches-ignore:
       - 'gh-readonly-queue/**'
+      - 'release-**'
   pull_request:
   merge_group:
   schedule: