ci: do not let dependabot automatically rebase

When dependabot creates a PR, and an other gets merged, the bot
automatically triggers a rebase. This will drop any approvals, causing
delays in the review/merge process.

The project uses Mergify to automatically rebase when needed, and
approvals are retained when Mergify rebases PR. By disabling the
auto-rebasing done by dependabot, fewer rebases should be needed,
contributors only need to review once, and CI jobs are triggered less
often.

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

.github/dependabot.yml

@@ -5,6 +5,7 @@ updates:
     directory: "/"
     schedule:
       interval: "weekly"
+    rebase-strategy: "disabled"
     labels:
       - rebase
     commit-message: