Replace camelCase format name dateOptionalTime

This format is deprecated and the snake case name date_optional_time
should be used instead. This mapping is already done on the server side
but trying to use the camelCase format produces a deprecation warning
as it's not allowed in ES 8+.

Change-Id: I6d6ba93e44d90f058a3c42857c7a082dd9e72d26
Release-Notes: skip

src/main/java/com/google/gerrit/elasticsearch/ElasticMapping.java

diff --git a/src/main/java/com/google/gerrit/elasticsearch/ElasticMapping.java b/src/main/java/com/google/gerrit/elasticsearch/ElasticMapping.java
index edd05c9..6bae325 100644
--- a/src/main/java/com/google/gerrit/elasticsearch/ElasticMapping.java
+++ b/src/main/java/com/google/gerrit/elasticsearch/ElasticMapping.java
@@ -23,7 +23,7 @@
 class ElasticMapping {
 
   protected static final String TIMESTAMP_FIELD_TYPE = "date";
-  protected static final String TIMESTAMP_FIELD_FORMAT = "dateOptionalTime";
+  protected static final String TIMESTAMP_FIELD_FORMAT = "date_optional_time";
 
   static MappingProperties createMapping(Schema<?> schema, ElasticQueryAdapter adapter) {
     ElasticMapping.Builder mapping = new ElasticMapping.Builder(adapter);