MFC r358554: Add deprecation notices to ctau and cx drivers

These support outdated or obsolete ISA WAN (T1/E1) sync serial cards,
and these drivers haven't really been touched (other than in tree-wide
sweeps to keep them building) for 15+ years.

Related PCI devices ce and cp are still in the tree, with deprecation
proposed in D23928.

Relnotes: Yes
Sponsored by: The FreeBSD Foundation