doc/go1.18: document FreeBSD 11.x will no longer be supported
doc/go1.18: document FreeBSD 11.x will no longer be supported

Mention future versions will require the COMPAT_FREEBSD12 kernel option
set in the kernel.

For #47694.

Change-Id: Ia94c4f9dbb38c68025d3c1d12dd2e241a5480a6b
Reviewed-on: https://go-review.googlesource.com/c/go/+/354971
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>

golang/go,nawawi/go,cloudflare/go,cloudflare/go,golang/go,cloudflare/go,nawawi/go,nawawi/go,CAFxX/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,golang/go,nawawi/go,golang/go,cloudflare/go,CAFxX/go,cloudflare/go,nawawi/go,golang/go,CAFxX/go,golang/go,CAFxX/go,CAFxX/go,CAFxX/go,cloudflare/go,CAFxX/go,nawawi/go,nawawi/go,golang/go