REST: deprecate `field_data` in Clear Cache API (#28943)

We call it `fielddata` everywhere else in the code and API so we may as
well be consistent.

rest-api-spec/src/main/resources/rest-api-spec/test/indices.clear_cache/10_basic.yml

@@ -25,3 +25,26 @@
         - 'Deprecated field [request_cache] used, expected [request] instead'
       indices.clear_cache:
         request_cache: false
+
+---
+"clear_cache with field_data set to true":
+  - skip:
+      version: " - 6.2.99"
+      reason: field_data was deprecated in 6.3.0
+      features: "warnings"
+
+  - do:
+      warnings:
+        - 'Deprecated field [field_data] used, expected [fielddata] instead'
+      indices.clear_cache:
+        field_data: true
+
+---
+"clear_cache with fielddata set to true":
+  - skip:
+      version: " - 6.2.99"
+      reason: fielddata was deprecated before 6.3.0
+
+  - do:
+      indices.clear_cache:
+        fielddata: true

server/src/main/java/org/elasticsearch/rest/action/admin/indices/RestClearIndicesCacheAction.java

@@ -88,7 +88,7 @@ public class RestClearIndicesCacheAction extends BaseRestHandler {
                 clearIndicesCacheRequest.queryCache(request.paramAsBoolean(entry.getKey(), clearIndicesCacheRequest.queryCache()));
             } else if (Fields.REQUEST.match(entry.getKey(), LoggingDeprecationHandler.INSTANCE)) {
                 clearIndicesCacheRequest.requestCache(request.paramAsBoolean(entry.getKey(), clearIndicesCacheRequest.requestCache()));
-            } else if (Fields.FIELD_DATA.match(entry.getKey(), LoggingDeprecationHandler.INSTANCE)) {
+            } else if (Fields.FIELDDATA.match(entry.getKey(), LoggingDeprecationHandler.INSTANCE)) {
                 clearIndicesCacheRequest.fieldDataCache(request.paramAsBoolean(entry.getKey(), clearIndicesCacheRequest.fieldDataCache()));
             } else  if (Fields.FIELDS.match(entry.getKey(), LoggingDeprecationHandler.INSTANCE)) {
                 clearIndicesCacheRequest.fields(request.paramAsStringArray(entry.getKey(), clearIndicesCacheRequest.fields()));
@@ -101,7 +101,7 @@ public class RestClearIndicesCacheAction extends BaseRestHandler {
     public static class Fields {
         public static final ParseField QUERY = new ParseField("query", "filter", "filter_cache");
         public static final ParseField REQUEST = new ParseField("request", "request_cache");
-        public static final ParseField FIELD_DATA = new ParseField("field_data", "fielddata");
+        public static final ParseField FIELDDATA = new ParseField("fielddata", "field_data");
         public static final ParseField FIELDS = new ParseField("fields");
     }