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
* @throws IOException if an IOException occurs
     */
    public Set<ShardId> findAllShardIds(final Index index) throws IOException {
        assert index != null;
        if (nodePaths == null || locks == null) {
            throw new IllegalStateException("node is not configured to store local location");
        }
        assertEnvIsLocked();
        final Set<ShardId> shardIds = new HashSet<>();
        final String indexUniquePathId = index.getUUID();
        for (final NodePath nodePath : nodePaths) {
            shardIds.addAll(findAllShardsForIndex(nodePath.indicesPath.resolve(indexUniquePathId), index));
        }
        return shardIds;
    }
    /**
     * Find all the shards for this index, returning a map of the {@code NodePath} to the number of shards on that path
     * @param index the index by which to filter shards
     * @return a map of NodePath to count of the shards for the index on that path
     * @throws IOException if an IOException occurs
     */
    public Map<NodePath, Long> shardCountPerPath(final Index index) throws IOException {
        assert index != null;
        if (nodePaths == null || locks == null) {
            throw new IllegalStateException("node is not configured to store local location");
        }
        assertEnvIsLocked();
        final Map<NodePath, Long> shardCountPerPath = new HashMap<>();
        final String indexUniquePathId = index.getUUID();
        for (final NodePath nodePath : nodePaths) {