fix #6728: remove mention of oldstable to clarify support/EOL policy

Someone found the original phrasing confusing, because a Debian
version stays stable and oldstable for roughly 4 years and Promox VE
is supported for about 3 years.

The bug report [0] also mentions a forum post [1] that should be
updated as well.

[0]: https://bugzilla.proxmox.com/show_bug.cgi?id=6728
[1]: https://forum.proxmox.com/threads/proxmox-ve-support-lifecycle.35755/

Signed-off-by: Robert Obkircher <r.obkircher@proxmox.com>
Link: https://lore.proxmox.com/all/20251103114055.34989-1-r.obkircher@proxmox.com

pve-faq.adoc

@@ -81,8 +81,9 @@ to run.
 How long will my {pve} version be supported?::
 
 {pve} versions are supported at least as long as the corresponding
-Debian Version is
-https://wiki.debian.org/DebianOldStable[oldstable]. {pve} uses a
+Debian version, i.e. approximately 3 years after its initial release,
+see https://www.debian.org/security/faq#lifespan[Debian lifespan].
+{pve} uses a
 rolling release model and using the latest stable version is always
 recommended.
 +