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/FeatureMigrationIT.java
@Override
    protected Settings nodeSettings(int nodeOrdinal, Settings otherSettings) {
        return Settings.builder().put(super.nodeSettings(nodeOrdinal, otherSettings)).build();
    }
    @Override
    protected boolean forbidPrivateIndexSettings() {
        
        return false;
    }
    @Override
    protected Collection<Class<? extends Plugin>> nodePlugins() {
        List<Class<? extends Plugin>> plugins = new ArrayList<>(super.nodePlugins());
        plugins.add(TestPlugin.class);
        plugins.add(ReindexPlugin.class);
        return plugins;
    }
    public void testStartMigrationAndImmediatelyCheckStatus() throws Exception {
        createSystemIndexForDescriptor(INTERNAL_MANAGED);