Fix main response to return build flavor (elastic#97857)

Build flavor was added back in elastic#97770, but the main endpoint response
is still hardcoded to "default". This commit updates the response to
return the flavor from the build info.

Author: rjernst

modules/rest-root/src/main/java/org/elasticsearch/rest/root/MainResponse.java

@@ -131,7 +131,7 @@ public XContentBuilder toXContent(XContentBuilder builder, Params params) throws
         builder.field("cluster_uuid", clusterUuid);
         builder.startObject("version")
             .field("number", build.qualifiedVersion())
-            .field("build_flavor", "default")
+            .field("build_flavor", build.flavor())
             .field("build_type", build.type().displayName())
             .field("build_hash", build.hash())
             .field("build_date", build.date())