bot: enable backports to release-1.8
bot: enable backports to release-1.8

With the creation of the release-1.8 branch we enable
the mergify bot to open the backport PRs automatically
based on the backport-release-1.8 label.

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

rook/rook,rook/rook,rook/rook,rook/rook,rook/rook