Use database dictionary to fetch gitlab schemas

What does this MR do and why?

We are deprecating the gitlab_schemas.yml file. In order to remove the gitlab_schemas.yml file, we have to fetch the gitlab_schema from the database dictionary. The gitlab_schema is a required field in the database dictionary.

Note:

The following classes are using .tables_to_schema:

https://gitlab.com/gitlab-org/gitlab/-/blame/cb909fb15cbc4d337b8c8ecae60c0048cacc1391/lib/gitlab/database/tables_truncate.rb#L23

https://gitlab.com/gitlab-org/gitlab/-/blame/cb909fb15cbc4d337b8c8ecae60c0048cacc1391/lib/tasks/gitlab/db/lock_writes.rake#L9

https://gitlab.com/gitlab-org/gitlab/-/blame/cb909fb15cbc4d337b8c8ecae60c0048cacc1391/spec/lib/gitlab/usage/service_ping_report_spec.rb#L115

I think we don't need to return views as well.

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

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