docs: add OMAP quick fix warning to upgrade guide

Ceph has recently reported that it may be unsafe to upgrade clusters
from Nautilus/Octopus to Pacific v16.2.0 through v16.2.6. We are
tracking this in Rook issue #9185.
Add a warning to the upgrade doc about this.

Signed-off-by: Blaine Gardner <blaine.gardner@redhat.com>

Documentation/ceph-upgrade.md

@@ -399,6 +399,10 @@ To migrate the setting, follow these steps:
 
 ## Ceph Version Upgrades
 
+> **WARNING: There is a notice from Ceph for users upgrading to Ceph Pacific v16.2.6 or lower from
+> an earlier major version of Ceph. If you are upgrading to Ceph Pacific (v16), please upgrade to
+> v16.2.7 or higher if possible. See for more: https://github.com/rook/rook/issues/9185**
+
 Rook v1.7 supports the following Ceph versions:
  - Ceph Pacific 16.2.0 or newer
  - Ceph Octopus v15.2.0 or newer