Use rebases instead of merge commits

Per #10048 (comment)
ff.

In studying the Mergify documentation, I discovered the actual rules are
slightly different than the ones we thought it was using, so I used the
ones the documentation cited (cf.
https://docs.mergify.com/workflow/actions/merge/#parameters).

We now use `squash` instead of `rebase`, so the PR number is preserved
for `changelog-d` to find.

.github/mergify.yml

@@ -19,10 +19,14 @@ pull_request_rules:
   - actions:
       queue:
         name: default
-        # Merge into master with a merge commit
-        method: merge
-        # Update the pr branch with rebase, so the history is clean
-        update_method: rebase
+        # Merge into master with a rebase
+        # NB. must use "squash" here so the squash commit contains the PR
+        # number fr changelog-d. If you really need it to not be squashed,
+        # use merge+no rebase.
+        method: squash
+        # both update methods get absorbed by the squash, so we use the most
+        # reliable
+        update_method: merge
     name: Put pull requests in the rebase+merge queue
     conditions:
       - base=master
@@ -60,8 +64,8 @@ pull_request_rules:
   - actions:
       queue:
         name: default
-        # Merge with a merge commit
-        method: merge
+        # Merge with a rebase
+        method: rebase
         # Update the pr branch with rebase, so the history is clean
         update_method: rebase
     name: Put backports in the rebase+merge queue