http: fix SYSLIB0057 warning for X509Certificate2Collection.Import

Use ImportFromPemFile on modern .NET to resolve the SYSLIB0057
deprecation warning, while keeping the original Import call on
.NET Framework where the new API is not available.

Signed-off-by: Matthew John Cheetham <mjcheetham@outlook.com>

Author: mjcheetham

src/shared/Core/HttpClientFactory.cs

@@ -130,7 +130,11 @@ public HttpClient CreateClient()
 
                     // Import the custom certs
                     X509Certificate2Collection certBundle = new X509Certificate2Collection();
+#if NETFRAMEWORK
                     certBundle.Import(certBundlePath);
+#else
+                    certBundle.ImportFromPemFile(certBundlePath);
+#endif
 
                     try
                     {