Schedule asynchronous index creation to support finding_id backfill

Changelog: added

db/post_migrate/20231130142218_async_create_supporting_index_for_finding_id_backfill.rb

+# frozen_string_literal: true
+
+class AsyncCreateSupportingIndexForFindingIdBackfill < Gitlab::Database::Migration[2.2]
+  milestone '16.7'
+
+  INDEX_NAME = "tmp_index_vulnerabilities_on_id_finding_id_empty"
+
+  # TODO: Index to be created synchronously in https://gitlab.com/gitlab-org/gitlab/-/issues/433253
+  def up
+    prepare_async_index(
+      :vulnerabilities,
+      :id,
+      where: "finding_id IS NULL",
+      name: INDEX_NAME
+    )
+  end
+
+  def down
+    unprepare_async_index(
+      :vulnerabilities,
+      :id,
+      name: INDEX_NAME
+    )
+  end
+end

db/schema_migrations/20231130142218

+2269867e97f1194f376979f964912f386aa5248966601a46e27ebb1b72d9e96a
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