Deprecate multimedia/gstreamer-plugins-buzztard

The module does not build with FluidSynth 2.x, is unused in the
ports tree, has been renamed to gst-buzztrax [0], which has been
discontinued upstream [1] in 2015.

While here update WWW.

[0] https://github.com/Buzztrax/gst-buzztrax/commit/10e18f63b513060b3ef657e493d75ac035bb2ded
[1] https://github.com/Buzztrax/gst-buzztrax/commit/3faf9eb12dd8cb7514650bd51f1f09fe590528d8

PR: 233501