doc: update new driver merge deadline
doc: update new driver merge deadline

Slightly change the text to indicate that it's M-2 of the current
release and link to the general releases page instead of a particular
release schedule.

Change-Id: I60e9ed7059088997c76d4ffb4e90e86805931958

openstack/cinder,mahak/cinder,mahak/cinder,openstack/cinder