Introduce :gitlab_geo schema for Geo tracking DB

What does this MR do and why?

Add a :gitlab_geo schema in lib/gitlab/database/gitlab_schemas.yml similar to the separation between :gitlab_main and :gitlab_ci to ensure our query analyzers catch any cross-joins and cross-transactions.

Related to #358422 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

ee/lib/ee/gitlab/database.rb

@@ -31,19 +31,10 @@ def database_base_models
           ).compact.with_indifferent_access.freeze
         end
 
-        override :database_base_models_using_load_balancing
-        def database_base_models_using_load_balancing
-          @database_base_models_using_load_balancing_ee ||=
-            super.select { |db_name, _| !geo_database?(db_name) }
-                 .with_indifferent_access
-                 .freeze
-        end
-
         override :schemas_to_base_models
         def schemas_to_base_models
           @schemas_to_base_models_ee ||= super.merge(
-            gitlab_geo: [self.database_base_models[:geo]].compact,
-            gitlab_shared: self.database_base_models.except(:geo).values # Geo does not support gitlab_shared tables
+            gitlab_geo: [self.database_base_models[:geo]].compact
           ).compact.with_indifferent_access.freeze
         end