uptime-kuma: pin floating ':2' -> 2.4.0

House style is a specific upstream tag. ':2' is a floating major that silently
moves under customers on every pull -- the same class of problem as ':latest',
just narrower. Pinned to the current 2.x release.

Campaign 7 catalog sweep.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Nn3VgQk9iwEGgyx6QJ2NvE

templates/uptime-kuma/docker-compose.yml

@@ -8,7 +8,7 @@
 
 services:
   uptime-kuma:
-    image: louislam/uptime-kuma:2
+    image: louislam/uptime-kuma:2.4.0
     container_name: uptime-kuma
     restart: unless-stopped
     environment: