Enable HNSW early termination by default in snapshot builds (elastic#139468)

This will allow us to measure activity across all our various benchmarks
and tests to make sure its all good before moving to switching the
default for this value to `true`.

Author: benwtrent

server/src/main/java/org/elasticsearch/index/mapper/vectors/DenseVectorFieldMapper.java

@@ -134,8 +134,6 @@ public class DenseVectorFieldMapper extends FieldMapper {
     public static final String COSINE_MAGNITUDE_FIELD_SUFFIX = "._magnitude";
     public static final int BBQ_MIN_DIMS = 64;
 
-    private static final boolean DEFAULT_HNSW_EARLY_TERMINATION = false;
-
     /**
      * The heuristic to utilize when executing a filtered search against vectors indexed in an HNSW graph.
      */
@@ -183,7 +181,7 @@ public KnnSearchStrategy getKnnSearchStrategy() {
 
     public static final Setting<Boolean> HNSW_EARLY_TERMINATION = Setting.boolSetting(
         "index.dense_vector.hnsw_enable_early_termination",
-        DEFAULT_HNSW_EARLY_TERMINATION,
+        Build.current().isSnapshot(),
         Setting.Property.IndexScope,
         Setting.Property.ServerlessPublic,
         Setting.Property.Dynamic