Deprecate template query.

Closes #19390

docs/reference/migration/migrate_5_0/scripting.asciidoc

@@ -309,4 +309,9 @@ transportClient.addTransportAddress(
 --------------------------------------------------
 
 Also the helper methods in `QueryBuilders` class that create a `TemplateQueryBuilder` instance have been removed,
-instead the constructors on `TemplateQueryBuilder` should be used.
+instead the constructors on `TemplateQueryBuilder` should be used.
+
+==== Template query
+
+The `template` query has been deprecated in favour of the search template api. The `template` query is scheduled
+to be removed in the next major version.

modules/lang-mustache/src/main/java/org/elasticsearch/script/mustache/TemplateQueryBuilder.java

@@ -23,6 +23,8 @@ import org.elasticsearch.common.ParsingException;
 import org.elasticsearch.common.bytes.BytesReference;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
+import org.elasticsearch.common.logging.DeprecationLogger;
+import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentFactory;
 import org.elasticsearch.common.xcontent.XContentParser;
@@ -47,8 +49,12 @@ import java.util.Optional;
 /**
  * Facilitates creating template query requests.
  * */
+@Deprecated
+// TODO remove this class in 6.0
 public class TemplateQueryBuilder extends AbstractQueryBuilder<TemplateQueryBuilder> {
+
     public static final String NAME = "template";
+    private static final DeprecationLogger DEPRECATION_LOGGER = new DeprecationLogger(Loggers.getLogger(TemplateQueryBuilder.class));
 
     /** Template to fill. */
     private final Script template;
@@ -63,6 +69,7 @@ public class TemplateQueryBuilder extends AbstractQueryBuilder<TemplateQueryBuil
 
     // for tests, so that mock script can be used:
     TemplateQueryBuilder(Script template) {
+        DEPRECATION_LOGGER.deprecated("[{}] query is deprecated, use search template api instead", NAME);
         if (template == null) {
             throw new IllegalArgumentException("query template cannot be null");
         }