Enable default ARIA for custom elements

https://bugs.webkit.org/show_bug.cgi?id=245532

Reviewed by Alexey Shvayka.

Enable this feature now that form-associated custom elements is enabled.

* Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml:

Canonical link: https://commits.webkit.org/258743@main

Author: rniwa

Source/WTF/Scripts/Preferences/UnifiedWebPreferences.yaml

@@ -1656,16 +1656,16 @@ DeclarativeShadowDOMEnabled:
 
 DefaultARIAForCustomElementsEnabled:
   type: bool
-  status: testable
+  status: stable
   humanReadableName: "Default ARIA for Custom Elements"
   humanReadableDescription: "Enable Default ARIA for Custom Elements"
   defaultValue:
     WebKitLegacy:
-      default: false
+      default: true
     WebKit:
-      default: false
+      default: true
     WebCore:
-      default: false
+      default: true
 
 DefaultFixedFontSize:
   type: double