C#: Update HttpServerUtility stub with HtmlEncode method and update flow summaries test.

csharp/ql/test/library-tests/dataflow/library/FlowSummaries.expected

@@ -2218,6 +2218,7 @@
 | System.Web.UI.WebControls;TextBox;false;get_Text;();;Argument[-1];ReturnValue;taint |
 | System.Web;HttpCookie;false;get_Value;();;Argument[-1];ReturnValue;taint |
 | System.Web;HttpCookie;false;get_Values;();;Argument[-1];ReturnValue;taint |
+| System.Web;HttpServerUtility;false;HtmlEncode;(System.String);;Argument[0];ReturnValue;taint |
 | System.Web;HttpServerUtility;false;UrlEncode;(System.String);;Argument[0];ReturnValue;taint |
 | System.Web;HttpUtility;false;HtmlAttributeEncode;(System.String);;Argument[0];ReturnValue;taint |
 | System.Web;HttpUtility;false;HtmlAttributeEncode;(System.String,System.IO.TextWriter);;Argument[0];ReturnValue;taint |

csharp/ql/test/library-tests/dataflow/library/FlowSummariesFiltered.expected

@@ -1903,6 +1903,7 @@
 | System.Web.UI.WebControls;TextBox;false;get_Text;();;Argument[-1];ReturnValue;taint |
 | System.Web;HttpCookie;false;get_Value;();;Argument[-1];ReturnValue;taint |
 | System.Web;HttpCookie;false;get_Values;();;Argument[-1];ReturnValue;taint |
+| System.Web;HttpServerUtility;false;HtmlEncode;(System.String);;Argument[0];ReturnValue;taint |
 | System.Web;HttpServerUtility;false;UrlEncode;(System.String);;Argument[0];ReturnValue;taint |
 | System.Web;HttpUtility;false;HtmlAttributeEncode;(System.String);;Argument[0];ReturnValue;taint |
 | System.Web;HttpUtility;false;HtmlAttributeEncode;(System.String,System.IO.TextWriter);;Argument[0];ReturnValue;taint |

csharp/ql/test/resources/stubs/System.Web.cs

@@ -42,6 +42,7 @@ namespace System.Web
     {
         public void Transfer(string path) { }
         public string UrlEncode(string s) => null;
+        public string HtmlEncode(string s) => null;
     }
 
     public class HttpApplication : IHttpHandler