cgroup: update note about legacy cgroup controller support

The 7->8 upgrade guide [1] mentions that cgroup v1 will be deprecated
starting from PVE 9.0, so also mention this in the docs.

[1] https://pve.proxmox.com/wiki/Upgrade_from_7_to_8#cgroup_V1_Deprecation

Signed-off-by: Friedrich Weber <[email protected]>

Author: frwbr

pct.adoc

@@ -722,9 +722,8 @@ environment, you can either
   decades old OS versions just fine there.
 
 * Switch back to the legacy 'cgroup' controller. Note that while it can be a
-  valid solution, it's not a permanent one. There's a high likelihood that a
-  future {pve} major release, for example 8.0, cannot support the legacy
-  controller anymore.
+  valid solution, it's not a permanent one. Starting from {pve} 9.0, the legacy
+  controller will not be supported anymore.
 ====
 
 [[pct_cgroup_change_version]]