doc: stop presenting EOL distros as current requirements

RHEL/CentOS 7.x and 8, Ubuntu 14.04/18.04, and CentOS 8.2 were still
stated as support floors in cephfs-top, the iSCSI gateway pages, the
OS recommendations kernel note, and the CloudStack guide. Restate
these against kernel versions or currently supported platforms.

Fixes: https://tracker.ceph.com/issues/79037

Signed-off-by: Emmanuel Ameh <eameh@contractor.linuxfoundation.org>