avoid using deprecated method when computing xml hash

VectoCommon/VectoHashing/Impl/XmlHashProvider.cs

@@ -93,6 +93,8 @@ namespace TUGraz.VectoHashing.Impl
 				throw new Exception($"CanonicalizationMethod(s) {string.Join(", ", unsupported)} not supported!");
 			}
 
+			// load any HMAC algorithm so that the key is also available. the HVAC algorithm used is the one set in the Reference object below
+			var hmac = HMAC.Create("HMACSHA256"); 
 			var signedXml = new SignedXml(doc);
 			var reference = new Reference("#" + elementId) {
 				DigestMethod = digestMethod
@@ -102,7 +104,7 @@ namespace TUGraz.VectoHashing.Impl
 			}
 
 			signedXml.AddReference(reference);
-			signedXml.ComputeSignature(HMAC.Create());
+			signedXml.ComputeSignature(hmac);
 			var xmlDigitalSignature = reference.GetXml();
 
 			var sigdoc = new XmlDocument();