Update HttpClient.DefaultProxy (dotnet#9095)

Fix HttpClient.DefaultProxy description.

- Remove incorrect `HttpWebRequest` reference to all HTTP requests (on the HttpClient)
- Change `HttpClientHandler` reference to `HttpMessageHandler`

Author: karelz

xml/System.Net.Http/HttpClient.xml

@@ -465,13 +465,13 @@ The specified `handler` will be disposed of by calling [HttpClient.Dispose](xref
         <ReturnType>System.Net.IWebProxy</ReturnType>
       </ReturnValue>
       <Docs>
-        <summary>Gets or sets the global Http proxy.</summary>
-        <value>A proxy used by every call that instantiates a <see cref="T:System.Net.HttpWebRequest" />.</value>
+        <summary>Gets or sets the global HTTP proxy.</summary>
+        <value>A proxy used by every HTTP request.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks
-This static property determines the default proxy that all <xref:System.Net.Http.HttpClient> instances use if no proxy is set explicitly in the <xref:System.Net.Http.HttpClientHandler> passed through its constructor. 
+This static property determines the default proxy that all <xref:System.Net.Http.HttpClient> instances use if no proxy is set explicitly in the <xref:System.Net.Http.HttpMessageHandler> passed through its constructor.
 
 The default instance returned by this property will initialize following a different set of rules depending on your platform:
 * **For Windows:** Reads proxy configuration from environment variables or, if those are not defined, from the user's proxy settings.