Make GDVIRTUAL Binding of _instantiate_playback non-const

[vivi512]

Fixes #112588 :

The GDVIRTUAL binding of the _instantiate_playback function of AudioStream marked it as const, when the actual function is non-const, making it impossible to override properly when using cpp bindings.

Binding (Incorrectly marked as const) :

godot/servers/audio/audio_stream.h

Line 173 in 6fd949a

GDVIRTUAL0RC_REQUIRED(Ref<AudioStreamPlayback>, _instantiate_playback)

Function(Non-const) :

godot/servers/audio/audio_stream.h

Line 185 in 6fd949a

virtual Ref<AudioStreamPlayback> instantiate_playback();

This PR fixes it (and the associated docs)

[Mickeon]

It may not seem like it but this requires proper handling because it technically breaks compatibility. See https://docs.godotengine.org/en/stable/engine_details/development/handling_compatibility_breakages.html .

Although, one could genuinely argue that because the function was never really... functioning as intended, the breakage is completely fine.

[dsnopek]

It shouldn't be too hard to add a compatibility method and not cause any breakage.

Are we really sure that no one is currently using this? It would be awkward, but folks could be using the current virtual function and casting const away from this:

Ref<AudioStreamPlayback> MyClass::_instantiate_playback() const {
  return const_cast<MyClass *>(this)->_instantiate_playback_nonconst();
}