[fix](backup) clear snapshot info for cancelled backup job to reduce …

…log size

Cherry-pick #32604

fe/fe-core/src/main/java/org/apache/doris/backup/BackupJob.java

@@ -859,6 +859,7 @@ private void cancelInternal() {
         }
 
         releaseSnapshots();
+        snapshotInfos.clear();
 
         BackupJobState curState = state;
         finishedTime = System.currentTimeMillis();