wscript: bump libplacebo minimum version

Switching to v3 allows us to drop the use of deprecated/removed members.

wscript

@@ -734,7 +734,7 @@ video_output_features = [
     }, {
         'name': '--libplacebo',
         'desc': 'libplacebo support',
-        'func': check_pkg_config('libplacebo >= 2.72.0'),
+        'func': check_pkg_config('libplacebo >= 3.104.0'),
     }, {
         'name': '--vulkan',
         'desc':  'Vulkan context support',