Deprecate postings highlighter

This change adds a deprecation warning for removal in 6.0.

Relates elastic#25028

core/src/main/java/org/elasticsearch/search/fetch/subphase/highlight/PostingsHighlighter.java

@@ -28,6 +28,8 @@
 import org.apache.lucene.search.highlight.Snippet;
 import org.apache.lucene.util.CollectionUtil;
 import org.elasticsearch.common.Strings;
+import org.elasticsearch.common.logging.DeprecationLogger;
+import org.elasticsearch.common.logging.Loggers;
 import org.elasticsearch.common.text.Text;
 import org.elasticsearch.index.mapper.FieldMapper;
 import org.elasticsearch.search.fetch.FetchPhaseExecutionException;
@@ -44,13 +46,15 @@
 import java.util.Locale;
 import java.util.Map;
 
+@Deprecated
 public class PostingsHighlighter implements Highlighter {
+    private static DeprecationLogger deprecationLogger = new DeprecationLogger(Loggers.getLogger(PostingsHighlighter.class));
 
     private static final String CACHE_KEY = "highlight-postings";
 
     @Override
     public HighlightField highlight(HighlighterContext highlighterContext) {
-
+        deprecationLogger.deprecated("[postings] highlighter is deprecated, please use [unified] instead");
         FieldMapper fieldMapper = highlighterContext.mapper;
         SearchContextHighlight.Field field = highlighterContext.field;
         if (canHighlight(fieldMapper) == false) {

docs/reference/search/request/highlighting.asciidoc

@@ -60,8 +60,13 @@ This is repeated for every field and every document that needs highlighting. If
 [[postings-highlighter]]
 ==== Postings highlighter
 
+deprecated[5.5.0,This highlighter type is deprecated and will be removed. Please use the `unified` highlighter instead. ]
+
+
 If `index_options` is set to `offsets` in the mapping the postings highlighter
-will be used instead of the plain highlighter. The postings highlighter:
+will be used instead of the plain highlighter.
+
+The postings highlighter:
 
 * Is faster since it doesn't require to reanalyze the text to be highlighted:
 the larger the documents the better the performance gain should be