Use rebases instead of merge commits

Per haskell#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).

.github/mergify.yml

@@ -19,8 +19,8 @@ pull_request_rules:
   - actions:
       queue:
         name: default
-        # Merge into master with a merge commit
-        method: merge
+        # Merge into master with a rebase
+        method: rebase
         # Update the pr branch with rebase, so the history is clean
         update_method: rebase
     name: Put pull requests in the rebase+merge queue
@@ -60,8 +60,8 @@ 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