[HLRC] change GET method to POST in Searchtemplate (#72851)

Fix #72073
Close #72397

client/rest-high-level/src/main/java/org/elasticsearch/client/RequestConverters.java

@@ -454,7 +454,7 @@ final class RequestConverters {
         } else {
             SearchRequest searchRequest = searchTemplateRequest.getRequest();
             String endpoint = endpoint(searchRequest.indices(), "_search/template");
-            request = new Request(HttpGet.METHOD_NAME, endpoint);
+            request = new Request(HttpPost.METHOD_NAME, endpoint);
 
             Params params = new Params();
             addSearchRequestParams(params, searchRequest);

client/rest-high-level/src/test/java/org/elasticsearch/client/RequestConvertersTests.java

@@ -1288,7 +1288,7 @@ public class RequestConvertersTests extends ESTestCase {
         }
         endpoint.add("_search/template");
 
-        assertEquals(HttpGet.METHOD_NAME, request.getMethod());
+        assertEquals(HttpPost.METHOD_NAME, request.getMethod());
         assertEquals(endpoint.toString(), request.getEndpoint());
         assertEquals(expectedParams, request.getParameters());
         assertToXContentBody(searchTemplateRequest, request.getEntity());