Deprecate variable-size receive predictor

With this commit we deprecate the following settings:

* http.netty.receive_predictor_min
* http.netty.receive_predictor_max

These settings are undocumented and will be removed with Elasticsearch
7.0.

Relates #26165

modules/transport-netty4/src/main/java/org/elasticsearch/http/netty4/Netty4HttpServerTransport.java

@@ -132,8 +132,18 @@ public class Netty4HttpServerTransport extends AbstractLifecycleComponent implem
 
     public static final Setting<ByteSizeValue> SETTING_HTTP_NETTY_RECEIVE_PREDICTOR_SIZE =
         Setting.byteSizeSetting("http.netty.receive_predictor_size", new ByteSizeValue(64, ByteSizeUnit.KB), Property.NodeScope);
+
+    /**
+     * @deprecated This (undocumented) setting is deprecated to reduce complexity and is removed in 7.0. See #26165 for details.
+     */
+    @Deprecated
     public static final Setting<ByteSizeValue> SETTING_HTTP_NETTY_RECEIVE_PREDICTOR_MIN =
         byteSizeSetting("http.netty.receive_predictor_min", SETTING_HTTP_NETTY_RECEIVE_PREDICTOR_SIZE, Property.NodeScope);
+
+    /**
+     * @deprecated This (undocumented) setting is deprecated to reduce complexity and is removed in 7.0. See #26165 for details.
+     */
+    @Deprecated
     public static final Setting<ByteSizeValue> SETTING_HTTP_NETTY_RECEIVE_PREDICTOR_MAX =
         byteSizeSetting("http.netty.receive_predictor_max", SETTING_HTTP_NETTY_RECEIVE_PREDICTOR_SIZE, Property.NodeScope);