Delete persisted cold cache index if created in the future (elastic#120465)

The Lucene index used to store cold cache files information (to persist the 
cold cache across node restarts) is upgraded at node startup, before the 
cluster is formed.

This upgrade can become an issue if the node cannot join the cluster: an 
attempt to downgrade the node back to the previous version will likely fail 
due to this Lucene index being already upgraded. Ideally we would prefer 
to upgrade the Lucene index after the node joined the cluster, but it requires 
more work as this Lucene index can be queried concurrently during shard 
allocation (see TransportSearchableSnapshotCacheStoresAction).

Instead of doing a more involved fix, this change deletes the persistent cache 
index from disk if it detects that the Lucene index has been upgraded. It assumes 
that we are already in a best effort downgrading procedure, so losing persistent 
cache is acceptable in order to allow downgrading the node.

Author: tlrx

x-pack/plugin/searchable-snapshots/src/main/java/org/elasticsearch/xpack/searchablesnapshots/cache/full/PersistentCache.java

@@ -437,6 +437,16 @@ static Map<String, Document> loadDocuments(Path directoryPath) throws IOExceptio
             } catch (IndexNotFoundException e) {
                 logger.debug("persistent cache index does not exist yet", e);
             }
+        } catch (Exception e) {
+            if (e instanceof IllegalArgumentException iae) {
+                final var message = iae.getMessage();
+                if (message != null && message.startsWith("indexCreatedVersionMajor is in the future:")) {
+                    logger.warn("Deleting persistent cache index created in the future [message: {}]", message);
+                    IOUtils.rm(directoryPath);
+                    return Map.of();
+                }
+            }
+            throw e;
         }
         return documents;
     }