Deprecate libsass

[bep]

The upstream library is now EOL and the repo is archived. Also, it's time for us to move away from the extended CGO build and for Hugo users not already done that, move to https://gohugo.io/functions/css/sass/#dart-sass

https://github.com/sass/libsass

[CamilleScholtz]

Note that Dart does not have any FreeBSD (and other BSD's) support, dropping libsass would effectively mean *BSD can not be used to host hugo/sass websites.

[jmooring]

@CamilleScholtz This will be addressed with sass/dart-sass#2413. Presuming we follow our standard deprecation practices, we will not disable the embedded LibSass transpiler for quite some time (approximately 12 months), giving the Sass team time to complete their efforts.

[bep]

We're also not in a big rush removing this, so we can certainly adjust the deprecation timeline; unfortunately the benefit of removing the extended build (with CGO) doesn't even closely match the cost of adding it in the first place.