Allow configuring POSTGRES_VERSION in Auto DevOps

We set the default value to 9.6.2 since this was the previous default
being used by `stable/postgresql --version=0.7.1` which is used in
auto-deploy-app.

changelogs/unreleased/58098-auto-devops-postgres-version-variable.yml

+---
+title: Allow configuring POSTGRES_VERSION in Auto DevOps
+merge_request: 25500
+author:
+type: added

doc/topics/autodevops/index.md

@@ -697,6 +697,7 @@ also be customized, and you can easily use a [custom buildpack](#custom-buildpac
 | `POSTGRES_USER`              | The PostgreSQL user; defaults to `user`. Set it to use a custom username.                                                                                                                                                     |
 | `POSTGRES_PASSWORD`          | The PostgreSQL password; defaults to `testing-password`. Set it to use a custom password.                                                                                                                                     |
 | `POSTGRES_DB`                | The PostgreSQL database name; defaults to the value of [`$CI_ENVIRONMENT_SLUG`](../../ci/variables/README.md#predefined-environment-variables). Set it to use a custom database name.                               |
+| `POSTGRES_VERSION`           | The PostgreSQL version; defaults to `9.6.2` |
 | `BUILDPACK_URL`              | The buildpack's full URL. It can point to either Git repositories or a tarball URL. For Git repositories, it is possible to point to a specific `ref`, for example `https://github.com/heroku/heroku-buildpack-ruby.git#v142` |
 | `SAST_CONFIDENCE_LEVEL`      | The minimum confidence level of security issues you want to be reported; `1` for Low, `2` for Medium, `3` for High; defaults to `3`.|
 | `DEP_SCAN_DISABLE_REMOTE_CHECKS` | Whether remote Dependency Scanning checks are disabled; defaults to `"false"`. Set to `"true"` to disable checks that send data to GitLab central servers. [Read more about remote checks](https://gitlab.com/gitlab-org/security-products/dependency-scanning#remote-checks).|

lib/gitlab/ci/templates/Auto-DevOps.gitlab-ci.yml

@@ -48,6 +48,7 @@ variables:
   POSTGRES_PASSWORD: testing-password
   POSTGRES_ENABLED: "true"
   POSTGRES_DB: $CI_ENVIRONMENT_SLUG
+  POSTGRES_VERSION: 9.6.2
 
   KUBERNETES_VERSION: 1.11.6
   HELM_VERSION: 2.12.2
@@ -700,6 +701,7 @@ rollout 100%:
         --set postgresql.postgresUser="$POSTGRES_USER" \
         --set postgresql.postgresPassword="$POSTGRES_PASSWORD" \
         --set postgresql.postgresDatabase="$POSTGRES_DB" \
+        --set postgresql.imageTag="$POSTGRES_VERSION" \
         --set application.initializeCommand="$DB_INITIALIZE" \
         --namespace="$KUBE_NAMESPACE" \
         "$name" \