Change default for vector.rescoring.directio to false (elastic#128784)

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);