Act on version 8 related asserts

[alpar-t]

We have a number of assertions for checking that we did not pump to version 8 yet.
I am in the process of muting them thus this issue to track them.
This is necessary so we can merge #38514 and get builds going.

• ResyncReplicationRequest, MetaDataCreateIndexService, RemoteClusterAware and RemoteClusterAware added by @jasontedor
• MetaStateService added by @andrershov

[elasticmachine]

Pinging @elastic/es-core-infra

[jasontedor]

@DaveCTurner Can you clean up the last remaining version 8 assertion in the code base? It's currently commented out:

elasticsearch/server/src/main/java/org/elasticsearch/gateway/MetaStateService.java

Lines 120 to 125 in a051321

	if (globalMetaData != null) {
	metaDataBuilder = MetaData.builder(globalMetaData);
	indexGraveyard = globalMetaData.custom(IndexGraveyard.TYPE);
	// TODO https://github.com/elastic/elasticsearch/issues/38556
	// assert Version.CURRENT.major < 8 : "failed to find manifest file, which is mandatory staring with Elasticsearch version 8.0";
	} else {

so there's some dead code there that theoretically can be cleaned up and removed.

[jasontedor]

There's an assertion in this block of code too:

elasticsearch/server/src/main/java/org/elasticsearch/gateway/MetaStateService.java

Lines 130 to 153 in a051321

	for (String indexFolderName : nodeEnv.availableIndexFolders()) {
	Tuple<IndexMetaData, Long> indexMetaDataAndGeneration =
	INDEX_META_DATA_FORMAT.loadLatestStateWithGeneration(logger, namedXContentRegistry,
	nodeEnv.resolveIndexFolder(indexFolderName));
	// TODO https://github.com/elastic/elasticsearch/issues/38556
	// assert Version.CURRENT.major < 8 : "failed to find manifest file, which is mandatory staring with Elasticsearch version 8.0";
	IndexMetaData indexMetaData = indexMetaDataAndGeneration.v1();
	long generation = indexMetaDataAndGeneration.v2();
	if (indexMetaData != null) {
	if (indexGraveyard.containsIndex(indexMetaData.getIndex())) {
	logger.debug("[{}] found metadata for deleted index [{}]", indexFolderName, indexMetaData.getIndex());
	// this index folder is cleared up when state is recovered
	} else {
	indices.put(indexMetaData.getIndex(), generation);
	metaDataBuilder.put(indexMetaData, false);
	}
	} else {
	logger.debug("[{}] failed to find metadata for existing index location", indexFolderName);
	}
	}
	Manifest manifest = Manifest.unknownCurrentTermAndVersion(globalStateGeneration, indices);
	return new Tuple<>(manifest, metaDataBuilder.build());
	}