ci: use ceph-csi-bot for Mergify actions

The `bot_account` option was not part of the Open Source plan, but now
it is. When Mergify rebases PRs, it sometimes leaves the following
message:

```
This pull request got rebased on behalf of a random user of the
organization. This behavior will change on the 1st February 2023,
Mergify will pick the author of the pull request instead.

To get the future behavior now, you can configure bot_account options
(e.g.: bot_account: { author } or update_bot_account: { author }.

Or you can create a dedicated github account for squash and rebase
operations, and use it in different bot_account options.
```

It is clearer for everyone when the Ceph-CSI Bot is used for automated
actions. Contributers might get confused why the PR was updated in their
name, without them doing anything.

Signed-off-by: Niels de Vos <ndevos@redhat.com>

.mergify.yml

@@ -1,9 +1,6 @@
 ---
 defaults:
   actions:
-    # mergify.io has removed bot_account from its free open source plan.
-    # comment:
-    # bot_account: ceph-csi-bot # mergify[bot] will be commenting.
     queue:
       # merge_bot_account: ceph-csi-bot #mergify[bot] will be merging prs.
       # update_bot_account: ceph-csi-bot #mergify will randomly pick and use
@@ -12,6 +9,9 @@ defaults:
       method: rebase
       rebase_fallback: merge
       update_method: rebase
+    rebase:
+      # Use ceph-csi-bot for rebasing, not the account of the PR owner.
+      bot_account: ceph-csi-bot
 
 queue_rules:
   - name: default