fix: add back compat functionality to the AudioBackend enum as well, and add missing migration comment from config version 70

src/Ryujinx/Systems/Configuration/AudioBackend.cs

@@ -10,5 +10,6 @@ namespace Ryujinx.Ava.Systems.Configuration
         OpenAl,
         SoundIo,
         SDL3,
+        SDL2 = SDL3
     }
 }

src/Ryujinx/Systems/Configuration/ConfigurationFileFormat.cs

@@ -15,7 +15,7 @@ namespace Ryujinx.Ava.Systems.Configuration
         /// <summary>
         /// The current version of the file format
         /// </summary>
-        public const int CurrentVersion = 70;
+        public const int CurrentVersion = 71;
 
         /// <summary>
         /// Version of the configuration file format

src/Ryujinx/Systems/Configuration/ConfigurationState.Migration.cs

@@ -484,7 +484,13 @@ namespace Ryujinx.Ava.Systems.Configuration
                     };
                 }
         ),
-                (69, static cff => cff.SkipUserProfiles = false)
+                (69, static cff => cff.SkipUserProfiles = false),
+                // no migration needed for 70
+                (71, static cff =>
+                {
+                    if (cff.AudioBackend is AudioBackend.SDL2)
+                        cff.AudioBackend = AudioBackend.SDL3;
+                })
             );
     }
 }