Added POSTGRES_VERSION variable to Auto DevOps Test stage

What does this MR do?

Fixes a bug where the POSTGRES_VERSION variable was ignored for the Auto DevOps Test stage.

Projects that use Auto DevOps and Postgres might experience failures due to a version mismatch, since it will no longer use latest but what version is set through the variable, or the default GitLab value (9.6.2).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers

Closes #11120 (closed)