Fix not-null inconsistency

Refer to https://gitlab.com/gitlab-org/gitlab/-/merge_requests/31886 for
details

This mostly aligns GitLab.com (and perhaps gdk environments) to use the
same `NOT NULL` constraints instead of `CHECK` constraints.

changelogs/unreleased/ab-fix-not-null-inconsistency.yml

+---
+title: Fix database schema inconsistency with not-null checks
+merge_request: 31930
+author:
+type: other

db/migrate/20200513160930_fix_not_null_check_constraint_inconsistency.rb

+# frozen_string_literal: true
+
+class FixNotNullCheckConstraintInconsistency < ActiveRecord::Migration[6.0]
+  include Gitlab::Database::MigrationHelpers
+
+  DOWNTIME = false
+
+  def up
+    table = :application_settings
+
+    %i(container_registry_vendor container_registry_version).each do |column|
+      change_column_null table, column, false
+      remove_not_null_constraint(table, column) if check_not_null_constraint_exists?(table, column)
+    end
+  end
+
+  def down
+    # No-op: for regular systems without the inconsistency, #up is a no-op, too
+  end
+end

db/structure.sql

@@ -13817,6 +13817,7 @@ COPY "schema_migrations" (version) FROM STDIN;
 20200511162115
 20200512085150
 20200512164334
+20200513160930
 20200513234502
 20200513235347
 20200513235532