Fix Backport Assistant PR commenting (#18200)

* Fix Backport Assistant failure PR commenting

For general comments on a PR, it looks like you have to use the `/issue`
endpoint rather than `/pulls`, which requires commit/other
review-specific target details.

This matches the endpoint used in `backport-reminder.yml`.

* Remove Backport Reminder workflow

This is noisy (even when adding multiple labels, individual comments per
label are generated), and likely no longer needed: we haven't had this
work in a long time due to an expired GH token, and we now have better
automation for backport PR assignment.

.github/workflows/backport-assistant.yml

@@ -40,4 +40,4 @@ jobs:
           curl -s -H "Authorization: token ${{ secrets.PR_COMMENT_TOKEN }}" \
             -X POST \
             -d "{ \"body\": \"${github_message}\"}" \
-            "https://api.github.com/repos/${GITHUB_REPOSITORY}/pull/${{ github.event.pull_request.number }}/comments"
+            "https://api.github.com/repos/${GITHUB_REPOSITORY}/issues/${{ github.event.pull_request.number }}/comments"