CI/GH actions: update find-comments version to v4

Author: dannywillems

.github/workflows/check-ocaml-refs.yaml

@@ -106,7 +106,7 @@ jobs:
 
       - name: Find existing OCaml validation comment
         if: github.event_name == 'pull_request'
-        uses: peter-evans/find-comment@v3
+        uses: peter-evans/find-comment@v4
         id: find-comment
         with:
           issue-number: ${{ github.event.pull_request.number }}