Database Dictionary: Migrate gitlab_schema from single yaml to docs files

Currently, we store information about what schema a database table should be in based on ./lib/gitlab/database/gitlab_schemas.yml, instead that should reside in the database dictionary files in db/docs/.

Definition of done

• Populate the gitlab_schema field for the database dictionary (gitlab-org/gitlab!103939 (merged))
• Add dictionary support for GEO (gitlab-org/gitlab!104429 (merged))
• Add gitlab_schema as a required field for the dictionary (gitlab-org/gitlab!104606 (merged))
• Change the dictionary structure to support database views (gitlab-org/gitlab!104855 (merged))
• Change the way we fetch GitLab schemas (gitlab-org/gitlab!105644 (merged))
• Remove the gitlab_schemas.yml file + update documentation (gitlab-org/gitlab!106811 (merged))

cc @ayufan @DylanGriffith