Add conflict_reminder action for backport PR

In order not to forget to resolve conflicts in backport PRs, we should add some reminders to the PR author so it will not be forgotten

the new action will run twice a week and will send a reminder only for
PR opened with conflicts for 3 days or more

Fixes: scylladb#21448

.github/workflows/conflict_reminder.yaml

@@ -0,0 +1,53 @@
+name: Notify PR Authors of Conflicts
+
+on:
+  schedule:
+    - cron: '0 0 * * *'  # Runs daily at midnight
+  workflow_dispatch:      # Manual trigger for testing
+
+jobs:
+  notify_conflict_prs:
+    runs-on: ubuntu-latest
+
+    steps:
+      - name: Notify PR Authors of Conflicts
+        uses: actions/github-script@v6
+        with:
+          script: |
+            const { owner, repo } = context.repo;
+            const conflictLabel = 'conflict';
+            const branchPrefix = 'branch-';
+            const daysOpenThreshold = 3;
+
+            // Get all open pull requests
+            const { data: pullRequests } = await github.pulls.list({
+              owner,
+              repo,
+              state: 'open',
+              per_page: 100,
+            });
+
+            const now = new Date();
+
+            for (const pr of pullRequests) {
+              const createdAt = new Date(pr.created_at);
+              const daysOpen = (now - createdAt) / (1000 * 60 * 60 * 24);
+
+              // Check if PR has the 'conflict' label, targets a branch with the specified prefix, and has been open for more than the threshold
+              const hasConflictLabel = pr.labels.some(label => label.name === conflictLabel);
+              const targetsBranchPrefix = pr.base.ref.startsWith(branchPrefix);
+
+              if (hasConflictLabel && targetsBranchPrefix && daysOpen > daysOpenThreshold) {
+                const author = pr.user.login;
+
+                // Post a comment on the PR to notify the author
+                await github.issues.createComment({
+                  owner,
+                  repo,
+                  issue_number: pr.number,
+                  body: `@${author}, this PR has been open with conflicts. Please resolve the conflicts to allow merging.`,
+                });
+
+                console.log(`Notified @${author} for PR #${pr.number}`);
+              }
+            }