bin/gen_release_notes: fix support for python 3.14

There is no default even loop anymore, we need to make one if we want
one now.

Cc: mesa-stable
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/39833>

bin/gen_release_notes.py

@@ -385,5 +385,5 @@ async def main() -> None:
 
 
 if __name__ == "__main__":
-    loop = asyncio.get_event_loop()
+    loop = asyncio.new_event_loop()
     loop.run_until_complete(main())