During upgrades from 7.x to 8.x if a node was holding a frozen cache file, the system considers that as an unknown file and prevents starting the node:
```
java.lang.IllegalStateException: unexpected file/folder encountered during data folder upgrade: /app/data/nodes/0/shared_snapshot_cache
	at org.elasticsearch.env.NodeEnvironment.upgradeLegacyNodeFolders(NodeEnvironment.java:443) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.env.NodeEnvironment.<init>(NodeEnvironment.java:290) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.node.Node.<init>(Node.java:388) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.node.Node.<init>(Node.java:277) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap$5.<init>(Bootstrap.java:234) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap.setup(Bootstrap.java:234) ~[elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Bootstrap.init(Bootstrap.java:358) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.init(Elasticsearch.java:166) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.execute(Elasticsearch.java:157) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.EnvironmentAwareCommand.execute(EnvironmentAwareCommand.java:77) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.Command.mainWithoutErrorHandling(Command.java:112) [elasticsearch-cli-8.0.1.jar:8.0.1]
	at org.elasticsearch.cli.Command.main(Command.java:77) [elasticsearch-cli-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:122) [elasticsearch-8.0.1.jar:8.0.1]
	at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:80) [elasticsearch-8.0.1.jar:8.0.1]
```
These files are expected files and should be ignored/accepted during the data folder upgrade.
server/src/main/java/org/elasticsearch/env/NodeEnvironment.java
Path indexLocation = nodePath.indicesPath.resolve(indexUniquePathId);
            if (Files.isDirectory(indexLocation)) {
                shardCountPerPath.put(nodePath, (long) findAllShardsForIndex(indexLocation, index).size());
            }
        }
        return shardCountPerPath;
    }
    private static Set<ShardId> findAllShardsForIndex(Path indexPath, Index index) throws IOException {
        assert indexPath.getFileName().toString().equals(index.getUUID());
        Set<ShardId> shardIds = new HashSet<>();
        if (Files.isDirectory(indexPath)) {
            try (DirectoryStream<Path> stream = Files.newDirectoryStream(indexPath)) {
                for (Path shardPath : stream) {
                    String fileName = shardPath.getFileName().toString();
                    if (Files.isDirectory(shardPath) && fileName.chars().allMatch(Character::isDigit)) {
                        int shardId = Integer.parseInt(fileName);
                        ShardId id = new ShardId(index, shardId);
                        shardIds.add(id);
                    }
                }
            }
        }
        return shardIds;
    }
    @Override
    public void close() {
        if (closed.compareAndSet(false, true) && locks != null) {
            for (Lock lock : locks) {
                try {
                    logger.trace("releasing lock [{}]", lock);