doc/start/os-recommendations: drop 4.14 kernel and reword guidance

The 4.14 LTS kernel has less than a year left in terms of maintenance,
drop it.

Also, the current wording with an explicit list of kernels tends to go
stale: it's missing the latest 6.1 LTS kernel.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

doc/start/os-recommendations.rst

@@ -18,12 +18,9 @@ Linux Kernel
   maintenance" kernel series provided by either http://kernel.org or
   your Linux distribution on any client hosts.
 
-  For RBD, if you choose to *track* long-term kernels, we currently recommend
-  4.x-based "longterm maintenance" kernel series or later:
-
-  - 4.19.z
-  - 4.14.z
-  - 5.x
+  For RBD, if you choose to *track* long-term kernels, we recommend
+  *at least* 4.19-based "longterm maintenance" kernel series.  If you can
+  use a newer "stable" or "longterm maintenance" kernel series, do it.
 
   For CephFS, see the section about `Mounting CephFS using Kernel Driver`_
   for kernel version guidance.