ci: add mergify backport target for release-1.4
ci: add mergify backport target for release-1.4

With the v1.4 release we will need mergify to support
the automatic backports based on the PR label.

Signed-off-by: Travis Nielsen <a8b474fdb3f77f44446f0ab9ddca8147b09b3a9e@redhat.com>

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