Pin PostgreSQL CI images to the latest versions

This should help avoid breaking changes made in the Docker images
(such as https://gitlab.com/gitlab-org/gitlab/issues/205820).

We also ship PostgreSQL 10.12 with Omnibus, so use this.

.gitlab/ci/global.gitlab-ci.yml

@@ -200,7 +200,7 @@
 
 .use-pg9:
   services:
-    - name: postgres:9.6
+    - name: postgres:9.6.17
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine
   variables:
@@ -209,7 +209,7 @@
 .use-pg10:
   image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6.5-golang-1.12-git-2.24-lfs-2.9-chrome-73.0-node-12.x-yarn-1.21-postgresql-10-graphicsmagick-1.3.34"
   services:
-    - name: postgres:10.9
+    - name: postgres:10.12
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine
   variables:
@@ -217,7 +217,7 @@
 
 .use-pg9-ee:
   services:
-    - name: postgres:9.6
+    - name: postgres:9.6.17
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine
     - name: elasticsearch:6.4.2
@@ -227,7 +227,7 @@
 .use-pg10-ee:
   image: "registry.gitlab.com/gitlab-org/gitlab-build-images:ruby-2.6.5-golang-1.12-git-2.24-lfs-2.9-chrome-73.0-node-12.x-yarn-1.21-postgresql-10-graphicsmagick-1.3.34"
   services:
-    - name: postgres:10.9
+    - name: postgres:10.12
       command: ["postgres", "-c", "fsync=off", "-c", "synchronous_commit=off", "-c", "full_page_writes=off"]
     - name: redis:alpine
     - name: elasticsearch:6.4.2