Remove setup_postgres task and update schema RB

What does this MR do?

Since we only support Postgresql since 12.1 (https://docs.gitlab.com/ee/install/requirements.html#database) I think we can remove the setup_postgresql rake task and move the indexes into schema.rb.

This will mean that developers will no-longer need to ignore changes like

t.index "lower((name)::text)", name: "index_projects_on_lower_name"

in their schema.rb file when committing.

Conformity

• Changelog entry
• Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content