pulseaudio: Revert "pulseaudio: Require PulseAudio 5.0 or later for SDL3."

This reverts commit 6fd0613ac8.

Turns out that the Steam Runtime is still on PulseAudio 1.1, and the only
thing we (currently) need a newer Pulse for is pa_threaded_mainloop_set_name,
so let's just go back to treating that symbol as optional.

We might need to force a higher version at some point, but it's not worth it
over this.

cmake/sdlchecks.cmake

@@ -162,7 +162,7 @@ endmacro()
 # - HAVE_SDL_LOADSO opt
 macro(CheckPulseAudio)
   if(SDL_PULSEAUDIO)
-    set(PulseAudio_PKG_CONFIG_SPEC "libpulse>=5.0")
+    set(PulseAudio_PKG_CONFIG_SPEC "libpulse>=0.9.15")
     pkg_check_modules(PC_PULSEAUDIO IMPORTED_TARGET ${PulseAudio_PKG_CONFIG_SPEC})
     if(PC_PULSEAUDIO_FOUND)
       set(HAVE_PULSEAUDIO TRUE)