Decrease page size for user list endpoint.

From some debugging via python-discord/kubernetes#96, we have determined
that this endpoint results in high latencies to the site and eventually
Kubernetes killing off site pods randomly.

Author: jchristgit

pydis_site/apps/api/viewsets/bot/user.py

@@ -19,7 +19,7 @@
 class UserListPagination(PageNumberPagination):
     """Custom pagination class for the User Model."""
 
-    page_size = 10000
+    page_size = 2500
     page_size_query_param = "page_size"
 
     def get_next_page_number(self) -> typing.Optional[int]: