Deprecate http.pipelining setting

This is related to elastic#29500. In 7.0 this setting will be removed. This PR
marks the setting in 6.x as deprecated.

server/src/main/java/org/elasticsearch/http/HttpTransportSettings.java

@@ -49,8 +49,9 @@ public final class HttpTransportSettings {
         new Setting<>("http.cors.allow-headers", "X-Requested-With,Content-Type,Content-Length", (value) -> value, Property.NodeScope);
     public static final Setting<Boolean> SETTING_CORS_ALLOW_CREDENTIALS =
         Setting.boolSetting("http.cors.allow-credentials", false, Property.NodeScope);
+    // In 7.0 pipelining support will always be enabled and this setting will be removed.
     public static final Setting<Boolean> SETTING_PIPELINING =
-        Setting.boolSetting("http.pipelining", true, Property.NodeScope);
+        Setting.boolSetting("http.pipelining", true, Property.NodeScope, Property.Deprecated);
     public static final Setting<Integer> SETTING_PIPELINING_MAX_EVENTS =
         Setting.intSetting("http.pipelining.max_events", 10000, Property.NodeScope);
     public static final Setting<Boolean> SETTING_HTTP_COMPRESSION =