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/AbstractFeatureMigrationIntegTest.java
.build();
    static final SystemIndexDescriptor INTERNAL_UNMANAGED = SystemIndexDescriptor.builder()
        .setIndexPattern(".int-unman-*")
        .setType(SystemIndexDescriptor.Type.INTERNAL_UNMANAGED)
        .setOrigin(ORIGIN)
        .setVersionMetaKey(VERSION_META_KEY)
        .setAllowedElasticProductOrigins(Collections.emptyList())
        .setMinimumNodeVersion(NEEDS_UPGRADE_VERSION)
        .setPriorSystemIndexDescriptors(Collections.emptyList())
        .build();
    static final SystemIndexDescriptor INTERNAL_MANAGED = SystemIndexDescriptor.builder()
        .setIndexPattern(".int-man-*")
        .setAliasName(".internal-managed-alias")
        .setPrimaryIndex(INTERNAL_MANAGED_INDEX_NAME)
        .setType(SystemIndexDescriptor.Type.INTERNAL_MANAGED)
        .setSettings(createSimpleSettings(NEEDS_UPGRADE_VERSION, INTERNAL_MANAGED_FLAG_VALUE))
        .setMappings(createSimpleMapping(true, true))
        .setOrigin(ORIGIN)
        .setVersionMetaKey(VERSION_META_KEY)
        .setAllowedElasticProductOrigins(Collections.emptyList())
        .setMinimumNodeVersion(NEEDS_UPGRADE_VERSION)
        .setPriorSystemIndexDescriptors(Collections.emptyList())
        .build();
    static final int INTERNAL_UNMANAGED_FLAG_VALUE = 2;
    static final int EXTERNAL_MANAGED_FLAG_VALUE = 3;
    static final SystemIndexDescriptor EXTERNAL_MANAGED = SystemIndexDescriptor.builder()
        .setIndexPattern(".ext-man-*")
        .setAliasName(".external-managed-alias")
        .setPrimaryIndex(".ext-man-old")
        .setType(SystemIndexDescriptor.Type.EXTERNAL_MANAGED)