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
/**
     * Checks to see if we can upgrade to this version based on the existing index state. Upgrading
     * from older versions can cause irreversible changes if allowed.
     * @param logger
     * @param nodePaths
     * @throws IOException
     */
    static void checkForIndexCompatibility(Logger logger, NodePath... nodePaths) throws IOException {
        final Path[] paths = Arrays.stream(nodePaths).map(np -> np.path).toArray(Path[]::new);
        NodeMetadata metadata = PersistedClusterStateService.nodeMetadata(paths);
        
        if (metadata == null) {
            throw new CorruptStateException(
                "Format version is not supported. Upgrading to ["
                    + Version.CURRENT
                    + "] is only supported from version ["
                    + Version.CURRENT.minimumCompatibilityVersion()
                    + "]."
            );
        }
        metadata.verifyUpgradeToCurrentVersion();
        logger.info("oldest index version recorded in NodeMetadata {}", metadata.oldestIndexVersion());
        if (metadata.oldestIndexVersion().isLegacyIndexVersion()) {
            throw new IllegalStateException(
                "cannot upgrade node because incompatible indices created with version ["
                    + metadata.oldestIndexVersion()
                    + "] exist, while the minimum compatible index version is ["
                    + Version.CURRENT.minimumIndexCompatibilityVersion()
                    + "]. "
                    + "Upgrade your older indices by reindexing them in version ["