Fix `SchemaCacheWithRenamedTable` does not work with many databases

Instead of getting the connection from ActiveRecord::Base,
we use the connection from ActiveRecord::ConnectionAdapters::SchemaCache.
Schema cache will return the proper connection based on the database context.

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/336899

lib/gitlab/database/schema_cache_with_renamed_table.rb

@@ -42,7 +42,7 @@ module Gitlab
       def renamed_tables_cache
         @renamed_tables ||= begin
           Gitlab::Database::TABLES_TO_BE_RENAMED.select do |old_name, new_name|
-            ActiveRecord::Base.connection.view_exists?(old_name)
+            connection.view_exists?(old_name)
           end
         end
       end