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).

.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