Mark field_data as deprecated in /_cache/clear

It was deprecated in #28943.

Author: nik9000

rest-api-spec/src/main/resources/rest-api-spec/api/indices.clear_cache.json

@@ -14,15 +14,15 @@
       "params": {
         "field_data": {
           "type" : "boolean",
-          "description" : "Clear field data"
+          "description" : "Clear field data. This is deprecated. Prefer `fielddata`."
         },
         "fielddata": {
           "type" : "boolean",
           "description" : "Clear field data"
         },
         "fields": {
           "type" : "list",
-          "description" : "A comma-separated list of fields to clear when using the `field_data` parameter (default: all)"
+          "description" : "A comma-separated list of fields to clear when using the `fielddata` parameter (default: all)"
         },
         "query": {
           "type" : "boolean",