Only run Geo migrations when database is PostgreSQL

We have quite a number of migrations on Geo tables. Most of them are built to run on MySQL too (not explicitly, but that's the default with migrations). But Geo is incompatible with MySQL, so it doesn't make sense to run those on MySQL.

Background

There was a ~"broken master" on some ~Geo specs just recently: https://gitlab.com/gitlab-org/gitlab-ee/issues/9503

Proposal

If we would write Gitlab::Database.postgresql? around all ~Geo migrations, we wouldn't need to make the migrations work on MySQL. And we would give the specs tag :postgresql so they would be run in MySQL builds, means they would reduce the build time.

We could apply this to all existing code, but as a first step I'd suggest to use this pattern at least for all new code/migrations.