Provisionally revert mergify.yml.

(to what it was before the label-related changes)

This is an experiment to see what happens in the CI run and not
expected to be merged.

.github/mergify.yml

@@ -8,7 +8,7 @@ pull_request_rules:
     conditions:
       - "#approved-reviews-by>=1"
       - check-success=mergify
-      - label="PR: ready to merge"
+      - label=ready-to-merge
     actions:
       update: {}
       queue:
@@ -17,12 +17,11 @@ pull_request_rules:
   - name: Delete head branch after merge
     conditions:
       - merged
-      # disabled October 2024
-      # - label!=keep-branch-after-merge
+      - label!=keep-branch-after-merge
     actions:
       delete_head_branch: {}
   - name: Keep pull requests with keep-updated label updated
     conditions:
-      - label="PR: keep updated"
+      - label=keep-updated
     actions:
       update: {}