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
);
        assertIndexHasCorrectProperties(
            finalMetadata,
            ".ext-man-old-reindexed-for-8",
            EXTERNAL_MANAGED_FLAG_VALUE,
            true,
            false,
            Arrays.asList(".ext-man-old", ".external-managed-alias")
        );
        assertIndexHasCorrectProperties(
            finalMetadata,
            ".ext-unman-old-reindexed-for-8",
            EXTERNAL_UNMANAGED_FLAG_VALUE,
            false,
            false,
            Collections.singletonList(".ext-unman-old")
        );
    }
    public void testMigrateIndexWithWriteBlock() throws Exception {
        createSystemIndexForDescriptor(INTERNAL_UNMANAGED);
        String indexName = Optional.ofNullable(INTERNAL_UNMANAGED.getPrimaryIndex())
            .orElse(INTERNAL_UNMANAGED.getIndexPattern().replace("*", "old"));
        client().admin().indices().prepareUpdateSettings(indexName).setSettings(Settings.builder().put("index.blocks.write", true)).get();
        TestPlugin.preMigrationHook.set((state) -> Collections.emptyMap());
        TestPlugin.postMigrationHook.set((state, metadata) -> {});
        ensureGreen();
        client().execute(PostFeatureUpgradeAction.INSTANCE, new PostFeatureUpgradeRequest()).get();
        assertBusy(() -> {
            GetFeatureUpgradeStatusResponse statusResp = client().execute(
                GetFeatureUpgradeStatusAction.INSTANCE,