fluidsynth: update from now deprecated api

fluidsynth: update from now deprecated api

fluid_synth_set_chorus_on and fluid_synth_set_reverb_on were
deprecated in favor of new funtions where you can also specify
the fx_group the effect would apply.

The behavior of the set_* variants was to apply to all groups
so we pass -1 to the new functions as per documentation.

https://www.fluidsynth.org/api/group__chorus__effect.html#ga3c48310eecdca9cd338799d19f19c32d

and

https://www.fluidsynth.org/api/group__reverb__effect.html#gacb7917564c988cf54f2e35189b509c8e

and the introduction of the change:

https://github.com/FluidSynth/fluidsynth/pull/673

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/2718>