There's some cleanup to be done in `FeatureMigrationIT` and related classes.

- [x] Convert `static` `preMigrationHook` and `postMigrationHook` to use instance members, retrieved from the `internalCluster()`
- [x] Use Hamcrest matchers as described [in this comment](https://github.com/elastic/elasticsearch/pull/84192/files#r812224067) for better failure output
- [x] Rename `createSimpleSettings` to `createSettings`
- [x] Rename `createSimpleMapping` to `createMapping`
- [x] add support for unknown fields https://github.com/elastic/elasticsearch/pull/84258#pullrequestreview-890890422
modules/reindex/src/internalClusterTest/java/org/elasticsearch/migration/MultiFeatureMigrationIT.java
/*
 * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
 * or more contributor license agreements. Licensed under the Elastic License
 * 2.0 and the Server Side Public License, v 1; you may not use this file except
 * in compliance with, at your election, the Elastic License 2.0 or the Server
 * Side Public License, v 1.
 */
package org.elasticsearch.migration;
import org.apache.lucene.util.SetOnce;
import org.elasticsearch.Version;
import org.elasticsearch.action.ActionListener;
import org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusAction;
import org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusRequest;
import org.elasticsearch.action.admin.cluster.migration.GetFeatureUpgradeStatusResponse;
import org.elasticsearch.action.admin.cluster.migration.PostFeatureUpgradeAction;
import org.elasticsearch.action.admin.cluster.migration.PostFeatureUpgradeRequest;
import org.elasticsearch.action.admin.cluster.migration.PostFeatureUpgradeResponse;
import org.elasticsearch.client.internal.Client;
import org.elasticsearch.cluster.ClusterState;
import org.elasticsearch.cluster.metadata.Metadata;
import org.elasticsearch.cluster.service.ClusterService;
import org.elasticsearch.common.Strings;
import org.elasticsearch.common.settings.Settings;
import org.elasticsearch.indices.SystemIndexDescriptor;
import org.elasticsearch.plugins.Plugin;
import org.elasticsearch.plugins.SystemIndexPlugin;
import org.elasticsearch.reindex.ReindexPlugin;
import org.elasticsearch.upgrades.FeatureMigrationResults;
import java.util.ArrayList;
import java.util.Arrays;