Simplify form title property assignment

Replaced `this.Text` with `Text` in the `UpdateGroupBoxTitle`
method to simplify code. This change removes the explicit use
of `this` while maintaining the same functionality.

Author: michaelmsonne

src/SignToolGUI/Forms/TimestampServerManagementForm.cs

@@ -32,12 +32,12 @@ private void UpdateGroupBoxTitle()
             if (_isTrustedSigning)
             {
                 groupBoxServers.Text = "Endpoints";
-                this.Text = "Trusted Signing Endpoint Management";
+                Text = "Trusted Signing Endpoint Management";
             }
             else
             {
                 groupBoxServers.Text = "Timestamp Servers";
-                this.Text = "Timestamp Server Management";
+                Text = "Timestamp Server Management";
             }
         }