Fix fieldnames field type for pre-6.1 indexes (#61322)

The FieldNamesFieldMapper field has different behaviour for indexes created in
clusters earlier than v6.1, and the code to deal with this was still using the vestigial
FieldType field of FieldMapper in its indexing path. This meant that documents
added after an upgrade were not correctly indexing their field names field. This
commit corrects the parseCreateField method to use the default field type.

Fixes #61305

Author: romseygeek

qa/full-cluster-restart/src/test/java/org/elasticsearch/upgrades/FullClusterRestartIT.java

@@ -98,7 +98,6 @@ public void setType() {
         type = getOldClusterVersion().before(Version.V_6_7_0) ? "doc" : "_doc";
     }
 
-    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/57245")
     public void testSearch() throws Exception {
         int count;
         if (isRunningAgainstOldCluster()) {

server/src/main/java/org/elasticsearch/index/mapper/FieldNamesFieldMapper.java

@@ -235,9 +235,7 @@ protected void parseCreateField(ParseContext context) throws IOException {
             }
             for (String path : paths) {
                 for (String fieldName : extractFieldNames(path)) {
-                    if (fieldType.indexOptions() != IndexOptions.NONE || fieldType.stored()) {
-                        document.add(new Field(fieldType().name(), fieldName, fieldType));
-                    }
+                    document.add(new Field(fieldType().name(), fieldName, Defaults.FIELD_TYPE));
                 }
             }
         }