increase version to 20.0.0 (see note below)

With start of Kodi 20 Nexus, takes addon as major the same version number as Kodi.
This done to know easier to which Kodi the addon works.

For here now used by Kodi 20 Nexus.

audiodecoder.sidplay/addon.xml.in

@@ -1,7 +1,7 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <addon
   id="audiodecoder.sidplay"
-  version="3.0.0"
+  version="20.0.0"
   name="Sidplay Audio Decoder"
   provider-name="spiff">
   <requires>@ADDON_DEPENDS@</requires>