Fix more key backup paths being unstable

Author: turt2live

src/client.ts

@@ -2926,7 +2926,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         const path = this.makeKeyBackupPath(roomId, sessionId, version);
         return this.http.authedRequest(
             undefined, Method.Put, path.path, path.queryData, data,
-            { prefix: PREFIX_UNSTABLE },
+            { prefix: PREFIX_V3 },
         );
     }
 
@@ -3217,7 +3217,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
 
             const res = await this.http.authedRequest<IRoomsKeysResponse | IRoomKeysResponse | IKeyBackupSession>(
                 undefined, Method.Get, path.path, path.queryData, undefined,
-                { prefix: PREFIX_UNSTABLE },
+                { prefix: PREFIX_V3 },
             );
 
             if ((res as IRoomsKeysResponse).rooms) {
@@ -3282,7 +3282,7 @@ export class MatrixClient extends TypedEventEmitter<EmittedEvents, ClientEventHa
         const path = this.makeKeyBackupPath(roomId, sessionId, version);
         return this.http.authedRequest(
             undefined, Method.Delete, path.path, path.queryData, undefined,
-            { prefix: PREFIX_UNSTABLE },
+            { prefix: PREFIX_V3 },
         );
     }