Update FAQ about kernel 5.15 in Turris OS 6.0

Section: ### Turris MOX / Turris Shield is not capable to route more than 450 Mbps

States: We are using the same LTS kernel versions as OpenWrt.

That was true before we released Turris OS 6.0 release. Since that, we are using kernel 5.15.

We need to reflect this change in user-docs.