[cinder][ops] restricting the Ceph version used with Cinder

The Cinder project plans to take advantage of recent developments in Ceph to make some improvements to the RBD driver. To do that, we propose restricting the Cinder-supported releases of Ceph to the active stable releases plus the two prior releases. This will make Mimic the minimum supported version of Ceph for the Wallaby and X releases of Cinder. This is explained in a bit more detail in a patch to the RBD driver documentation: https://review.opendev.org/#/c/762592/ If you have concerns about this proposal, please leave comments on the review. Thank you, brian