bot: mergify add release 1.5
bot: mergify add release 1.5

Update mergify to auto backport PR with label on release 1.5 branch and
auto merge.

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

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