Backport of Fix Backport Assistant PR commenting into release/1.15.x (#18202)

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.

Manual backport of 1c7fcdf188.

.github/workflows/backport-assistant.yml

@@ -64,4 +64,4 @@ 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"

.github/workflows/backport-reminder.yml

@@ -1,26 +0,0 @@
-# This workflow sends a reminder comment to PRs that have labels starting with
-# `backport/` to check that the backport has run successfully.
-
-name: Backport Assistant Reminder
-
-on:
-  pull_request:
-    types: [ labeled ]
-    # Runs on PRs to main and all release branches
-    branches:
-      - main
-      - release/*
-
-jobs:
-  backport-label-check:
-    if: "startsWith(github.event.label.name, 'backport/')"
-    runs-on: ubuntu-latest
-
-    steps:
-      - name: Comment on PR
-        run: |
-          github_message="After merging, confirm that you see linked PRs AND check them for CI errors."
-          curl -s -H "Authorization: token ${{ secrets.PR_COMMENT_TOKEN }}" \
-            -X POST \
-            -d "{ \"body\": \"${github_message}\"}" \
-            "https://api.github.com/repos/${GITHUB_REPOSITORY}/issues/${{ github.event.pull_request.number }}/comments"