Subsonic: Added deprecation notice for funkwhaleVersion field

Part-of: <https://dev.funkwhale.audio/funkwhale/funkwhale/-/merge_requests/2695>

api/funkwhale_api/subsonic/renderers.py

@@ -35,6 +35,8 @@ ET._serialize_xml = ET._serialize["xml"] = _serialize_xml
 
 def structure_payload(data):
     payload = {
+        # funkwhaleVersion is deprecated and will be removed in a future
+        # release. Use serverVersion instead.
         "funkwhaleVersion": funkwhale_api.__version__,
         "serverVersion": funkwhale_api.__version__,
         "status": "ok",

changes/changelog.d/subsonic-funkwhale-version.deprecation

@@ -0,0 +1,2 @@
+The "funkwhaleVersion" field in Subsonic responses is deprecated. Clients
+should use the OpenSubsonic field "serverVersion" instead.