Update postgres Docker tag to v18

Author: renovate[bot]

.github/workflows/test.yml

@@ -21,7 +21,7 @@ jobs:
 
     services:
       postgres:
-        image: postgres:16
+        image: postgres:18
         ports:
         - 5432:5432
         env:

docker-compose-prod.yml

@@ -67,7 +67,7 @@ services:
     command: memcached -I 8M
 
   db:
-    image: postgres:16
+    image: postgres:18
     container_name: th_db
     restart: always
     environment:

docker-compose.yml

@@ -66,7 +66,7 @@ services:
     command: memcached -I 8M
 
   db:
-    image: postgres:16
+    image: postgres:18
     container_name: th_db
     environment:
       # Relevant only when the container finds no existing database.