CNDB-12526 move PR checklist into PR comment (#1541)

Having checklist in the PR template creates an issue with copying PR
description into squashed commit:
- either commit messages are polluted with PR templates by default
- or commit message contains only title by default.

This PR moves checklist into separate comment, so both title and
description of a PR can be copied into squashed commit message by
default.

.github/workflows/checklist_comment_on_new_pr.yml

@@ -0,0 +1,18 @@
+name: Comment on new Pull Request with checklist
+on:
+  pull_request:
+    types: opened
+
+jobs:
+  checklist-comment:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout Repository
+        uses: actions/checkout@v4
+      - name: Add comment
+        run:
+          gh pr comment $PRNUM --body-file .github/workflows/pr_checklist.md
+        env:
+          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
+          GH_REPO: ${{ github.repository }}
+          PRNUM: ${{ github.event.pull_request.number }}

.github/workflows/pr_checklist.md

@@ -0,0 +1,10 @@
+### Checklist before you submit for review
+- [ ] Make sure there is a PR in the CNDB project updating the Converged Cassandra version
+- [ ] Use `NoSpamLogger` for log lines that may appear frequently in the logs
+- [ ] Verify test results on Butler
+- [ ] Test coverage for new/modified code is > 80%
+- [ ] Proper code formatting
+- [ ] Proper title for each commit staring with the project-issue number, like CNDB-1234
+- [ ] Each commit has a meaningful description
+- [ ] Each commit is not very long and contains related changes
+- [ ] Renames, moves and reformatting are in distinct commits