chore(deps): update postgres docker tag to v18 (#2692)

Author: renovate-bot

getting-started/assets/postgres/docker-compose-postgres.yml

@@ -19,7 +19,7 @@
 
 services:
   postgres:
-    image: postgres:17.6
+    image: postgres:18.0
     ports:
       - "5432:5432"
     # set shared memory limit when using docker-compose

runtime/admin/src/testFixtures/resources/org/apache/polaris/admintool/Dockerfile-postgres-version

@@ -19,4 +19,4 @@
 
 # Dockerfile to provide the image name and tag to a test.
 # Version is managed by Renovate - do not edit.
-FROM postgres:17.6-alpine
+FROM postgres:18.0-alpine