Change default for vector.rescoring.directio to false

On serverless (and potentially elsewhere), direct IO is not available,
which can cause BBQ shards to fail to read with
org.apache.lucene.CorruptIndexException when this setting is true.

Author: dpifke-elastic

server/src/main/java/org/elasticsearch/index/codec/vectors/es818/DirectIOLucene99FlatVectorsReader.java

@@ -57,7 +57,7 @@
 @SuppressForbidden(reason = "Copied from lucene")
 public class DirectIOLucene99FlatVectorsReader extends FlatVectorsReader implements OffHeapStats {
 
-    private static final boolean USE_DIRECT_IO = Boolean.parseBoolean(System.getProperty("vector.rescoring.directio", "true"));
+    private static final boolean USE_DIRECT_IO = Boolean.parseBoolean(System.getProperty("vector.rescoring.directio", "false"));
 
     private static final long SHALLOW_SIZE = RamUsageEstimator.shallowSizeOfInstance(DirectIOLucene99FlatVectorsReader.class);