RecoveryMonitor#lastSeenAccessTime should be volatile (#36781)

This local field is accessed on multiple threads and is nonvolatile so
theoretically could yield stale values. Not sure it does in practice.

server/src/main/java/org/elasticsearch/indices/recovery/RecoveriesCollection.java

@@ -258,7 +258,7 @@ private class RecoveryMonitor extends AbstractRunnable {
         private final long recoveryId;
         private final TimeValue checkInterval;
 
-        private long lastSeenAccessTime;
+        private volatile long lastSeenAccessTime;
 
         private RecoveryMonitor(long recoveryId, long lastSeenAccessTime, TimeValue checkInterval) {
             this.recoveryId = recoveryId;