mergify: change config to allow auto-merging
mergify: change config to allow auto-merging

Let's not do a rebase, but only merge. Rook is configured to refuse the
rebase and mergify fails with the following Github error:

GitHub error message: Rebase merges are not allowed on this repository.

This commit fixes the error.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

rook/nfs,rook/rook,rook/rook,rook/nfs,rook/rook,rook/rook,rook/nfs,rook/rook