Minimally require PostgreSQL 10

Currently, the minimum required postgres version for GitLab is 9.6 as per https://docs.gitlab.com/ee/install/requirements.html#postgresql-requirements:

As of GitLab 10.0, PostgreSQL 9.6 or newer is required, and earlier versions are not supported. We highly recommend users to use PostgreSQL 9.6 as this is the PostgreSQL version used for development and testing.

When we drop MySQL support and moved over to a plain SQL schema file, we can start using advanced postgres features.

As an example, we are greatly going to benefit from Postgres' table partitioning feature. Declarative partitioning has been introduced with Postgres 10 and is even more powerful in Postgres 11.

With that in mind, we should consider making >= Postgres 10 as a minimum database requirement for GitLab. In the bast case, we'd make it >= Postgres 11, which allows us to use the latest features but is more aggressive.

This issue is for discussion and actually bumping the version.

Note that for GitLab.com, we plan to upgrade to Postgres 11 soon.

Currently, we ship Postgres 10 with gitlab-omnibus.