syscons(4): add deprecation notice

[imp]

Yea! This is one of the last huge users of Giant. vt(4) was the rewrite to get locking right.
The one issue that has been flagged privately is the one around needing this for nvidia, so we should see if that's still the case (the person mentioning it to me started with 'it used to be the case... but I don't have that hardware anymore).

[emaste]

A couple of issues have been highlighted, e.g. there are some users looking for non-UTF-8 consoles still. My hope is that the deprecation notice helps encourage users to report any currently-unknown issues.

Currently known issues can be found via the Newcons (ugh) wiki page and bugzilla vt keyword search.

• https://wiki.freebsd.org/Newcons
• https://bugs.freebsd.org/bugzilla/buglist.cgi?keywords=vt&list_id=750929&resolution=---

[vova_fbsd.ru]

vt has serious flow if compare with sc: it is not possible to switch it to any mode except 80x25 in some virtual environments

I guess it is highlited at https://wiki.freebsd.org/Newcons
as "Limited functionality (mode switch only by loader)"
in my case - it is not possible even switch in loader,
so only option available with vt is 80x25 text screen
while with sc + vesa - it works fine in MODE_378

% vidcontrol -i adapter < /dev/ttyv0
fb0:
    vga0, type:VESA VGA (5), flags:0x700ff
    initial mode:24, current mode:378, BIOS mode:3
    frame buffer window:0xfd000000, buffer size:0x3e8000
    window size:0x3e8000, origin:0x0
    display start address (0, 0), scan line width:5120
    reserved:0xfd000000
% vidcontrol -i mode < /dev/ttyv0 | wc -l
     109
% sysctl kern.vty
kern.vty: sc
% fgrep sc0 /var/run/dmesg.boot
sc0: <System console> at flags 0x100 on isa0
sc0: VGA <16 virtual consoles, flags=0x300>
sc0: non-PNP ISA device will be removed from GENERIC in FreeBSD 15.

It would be really pity to loose this ability, nowdays it is quite inconvinient do whatever through window 80x25
VM is very basic - CX23 on Hetzner