Run gitlab-org/gitlab rspec pg tests against pg9

Objective

Align MR testing strategy with current GitLab.com to ensure availability for changes of GitLab.com.

Plan

• MR pipelines for gitlab-org/gitlab will use pg9 instead of pg11
• Master pipelines for gitlab-org/gitlab will use pg9 instead of pg10
• Scheduled master pipelines (every 2 hours and nightly) will remain running with pg9, pg10, pg11
• Update docs at https://docs.gitlab.com/ee/development/pipelines.html#postgresql-versions-testing

Where?	PG version
MRs	9.6
master (non-scheduled pipelines)	9.6 / 11
2-hourly scheduled pipelines	9.6 / 11
Nightly scheduled pipeline	9.6 / 10