Add `namespace_details.deleted_at` column

We're transitioning from the boolean `namespace_details.pending_delete`
column to the new `deleted_at` timestamp column. This change improves
clarity, as `pending_delete` incorrectly suggests future scheduling
rather than an ongoing process.

The `deleted_at` column will now indicate when the background deletion
process for a group has started. This will also enable us to see when
the group was deleted to help in debugging if the group deletion fails.

Changelog: other

db/migrate/20241029063914_add_deleted_at_to_namespace_details.rb

+# frozen_string_literal: true
+
+class AddDeletedAtToNamespaceDetails < Gitlab::Database::Migration[2.2]
+  milestone '17.6'
+
+  def change
+    add_column :namespace_details, :deleted_at, :datetime_with_timezone
+  end
+end

db/schema_migrations/20241029063914

+ef347416a77ffd7337d9d38ed191a18920e990bc168738a4058826c5113d7d61
\ No newline at end of file

db/structure.sql

@@ -14474,7 +14474,8 @@ CREATE TABLE namespace_details (
     description text,
     description_html text,
     creator_id bigint,
-    pending_delete boolean DEFAULT false NOT NULL
+    pending_delete boolean DEFAULT false NOT NULL,
+    deleted_at timestamp with time zone
 );
 CREATE TABLE namespace_import_users (