Increased the maximum number of channels in SoundTouch from 16ch to 32ch

[FranceBB]

• Increased the max number of channels to 32ch from the old limit of 16ch.
• Removed the hardcoded 16ch check in FIRFilter.cpp to make it use the SOUNDTOUCH_MAX_CHANNELS variable defined in STTypes.h instead.

Both of those changes follow what was actually committed by Olli Parviainen, the SoundTouch maintainer, in the main repo: https://codeberg.org/soundtouch/soundtouch/commit/ddf28667c9f52f30573853c8177e77149829fa7c

This was tracked here as part of this conversation: https://codeberg.org/soundtouch/soundtouch/issues/38