Increase `http.max_header_size` default to 16kb (#88725)

Our current default for the http.max_header_size setting is 8kb. This
is lower than the current default for Kibana (16kb in 8.x), and the ESS
proxy (1mb based on the Go http library default). To align with the
current convention of other Elastic components, this PR increases the
ES header size setting default to 16kb.

Closes #88501

docs/changelog/88725.yaml

@@ -0,0 +1,6 @@
+pr: 88725
+summary: Increase `http.max_header_size` default to 16kb
+area: Network
+type: enhancement
+issues:
+ - 88501

docs/reference/modules/http.asciidoc

@@ -51,7 +51,7 @@ Maximum size of an HTTP URL. Defaults to `4kb`.
 
 `http.max_header_size`::
 (<<static-cluster-setting,Static>>)
-Maximum size of allowed headers. Defaults to `8kb`.
+Maximum size of allowed headers. Defaults to `16kb`.
 
 [[http-compression]]
 // tag::http-compression-tag[]

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

@@ -106,7 +106,7 @@ public final class HttpTransportSettings {
     );
     public static final Setting<ByteSizeValue> SETTING_HTTP_MAX_HEADER_SIZE = Setting.byteSizeSetting(
         "http.max_header_size",
-        new ByteSizeValue(8, ByteSizeUnit.KB),
+        new ByteSizeValue(16, ByteSizeUnit.KB),
         Property.NodeScope
     );
     public static final Setting<Integer> SETTING_HTTP_MAX_WARNING_HEADER_COUNT = intSetting(