Update DigestMethod default value to SHA256 (#4320)

Reference default DigestMethod has been changed to SHA256 in this commit: dotnet/runtime@f628235
Documentation still displays SHA1 as default value.

Author: codernr

xml/System.Security.Cryptography.Xml/Reference.xml

@@ -284,14 +284,14 @@
       </ReturnValue>
       <Docs>
         <summary>Gets or sets the digest method Uniform Resource Identifier (URI) of the current <see cref="T:System.Security.Cryptography.Xml.Reference" />.</summary>
-        <value>The digest method URI of the current <see cref="T:System.Security.Cryptography.Xml.Reference" />. The default value is <c>http://www.w3.org/2000/09/xmldsig#sha1</c>.</value>
+        <value>The digest method URI of the current <see cref="T:System.Security.Cryptography.Xml.Reference" />. The default value is <c>http://www.w3.org/2001/04/xmlenc#sha256</c>.</value>
         <remarks>
           <format type="text/markdown"><![CDATA[  
   
 ## Remarks  
  The <xref:System.Security.Cryptography.Xml.Reference.DigestMethod%2A> property uses a URI string to represent the `<DigestMethod>` element of an XML digital signature.  
   
- The digest method is the algorithm used to hash the <xref:System.Security.Cryptography.Xml.Reference>. The default algorithm is <xref:System.Security.Cryptography.SHA1>.  
+ The digest method is the algorithm used to hash the <xref:System.Security.Cryptography.Xml.Reference>. The default algorithm is <xref:System.Security.Cryptography.SHA256>.  
   
  Due to collision problems with SHA1, Microsoft recommends a security model based on SHA256 or better. For more information about XML digital signatures, see the [W3C specification](https://www.w3.org/TR/xmldsig-core/).