nixos/tests/matrix/{conduit,dendrite}: fix deprecated asyncio usage

`asyncio.get_event_loop()` was deprecated in python 3.14,
see https://docs.python.org/3/whatsnew/3.14.html#id10 for more info.

nixos/tests/matrix/conduit.nix

@@ -77,7 +77,7 @@ in
                 # Close the client
                 await client.close()
 
-            asyncio.get_event_loop().run_until_complete(main())
+            asyncio.run(main())
           '')
         ];
       };

nixos/tests/matrix/dendrite.nix

@@ -77,7 +77,7 @@ in
                 # Close the client
                 await client.close()
 
-            asyncio.get_event_loop().run_until_complete(main())
+            asyncio.run(main())
           '')
         ];
       };