meetups: update London UG link (#1935)

Update the URL as Eventbrite is no longer used.
See https://discourse.nixos.org/t/london-nix-user-group/27825/65

Author: kerfuzzle

core/src/content/community/meetups.yaml

@@ -14,7 +14,7 @@ europe:
     location: Braunschweig (Germany)
   - href: https://www.meetup.com/Greater-Copenhagen-NixOS-User-Group
     location: Copenhagen (Denmark)
-  - href: https://www.eventbrite.co.uk/e/london-nix-user-group-tickets-729796249227
+  - href: https://discourse.nixos.org/t/london-nix-user-group/27825
     location: London (United Kingdom)
   - href: https://mobilizon.fr/@nix-mtp
     location: Montpellier (France)