Replace Deprecated_Header in navigator.platform doc

For the navigator.platform article, this change replaces the
Deprecated_Header macro and its boilerplate text with some
more-appropriate alternative pseudo-boilerplate text.

See #14429

files/en-us/web/api/navigator/platform/index.md

@@ -11,7 +11,9 @@ tags:
   - platform
 browser-compat: api.Navigator.platform
 ---
-{{ APIRef("HTML DOM") }} {{Deprecated_Header}}
+{{ APIRef("HTML DOM") }}
+
+> **Warning:** Use of this feature is discouraged. It may have known problems, or it may be a feature that works as expected now, but for which in the future behavior may change in a way that will break code that relies on it continuing to work in the same way it does now. Or it may be a feature which a specification has marked as deprecated or obsolete. Avoid using it, and update existing code if possible; see the [compatibility table](#browser_compatibility) to guide your decision.
 
 > **Note:** An alternative to this property is {{domxref("NavigatorUAData.platform", "navigator.userAgentData.platform")}}. However, {{domxref("NavigatorUAData.platform", "navigator.userAgentData.platform")}} is not yet supported by most browsers, and the specification which defines it has not yet been adopted by any standards group (specifically, it is not part of any specification published by the W3C or WHATWG).