Similarly to https://github.com/elastic/elasticsearch/pull/97873 but for request handlers: let's stop passing around strings and looking these things up by name on each request when we could just pass around the `Executor` itself.
server/src/main/java/org/elasticsearch/action/support/broadcast/unpromotable/TransportBroadcastUnpromotableAction.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License