Add `gitlab_schema` as a required field for the database dictionary

What does this MR do and why?

In order to deprecate the gitlab_schemas.yml file, we need to have a gitlab_schema attribute in the database dictionary.

In this MR, we are doing the following operations:

• Add gitlab_schema as a required field
• Change documentation
• Populate old entries

gitlab-org/database-team/team-tasks#260 (closed)

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