Sitemap.xml "lastmod" had erroneous datetimes in some locales. (#8105)

Sitemap.xml "lastmode" had erroneous datetimes in some locales.

In 'fi' CultureInfo DateTime.Now.ToString("yyyy-MM-ddThh:mm:ssK") returns "2022-07-25T10.21.47+03:00". Colons were converted into dots '.'.

Google did not index the site due to datetime errors.

Co-authored-by: Toni Kalajainen <toni.kalajainen@iki.fi>

Author: tagcode

src/Microsoft.DocAsCode.Build.Engine/PostProcessors/SitemapGenerator.cs

@@ -7,6 +7,7 @@ namespace Microsoft.DocAsCode.Build.Engine
     using System.Collections.Generic;
     using System.Collections.Immutable;
     using System.Composition;
+    using System.Globalization;
     using System.IO;
     using System.Linq;
     using System.Xml.Linq;
@@ -115,7 +116,7 @@ private XElement GetElement(string relativePath, Uri baseUri, SitemapElementOpti
             return new XElement
                  (Namespace + "url",
                  new XElement(Namespace + "loc", uri.AbsoluteUri),
-                 new XElement(Namespace + "lastmod", (options.LastModified ?? DateTime.Now).ToString("yyyy-MM-ddThh:mm:ssK")),
+                 new XElement(Namespace + "lastmod", (options.LastModified ?? DateTime.Now).ToString("yyyy-MM-ddThh:mm:ssK", CultureInfo.InvariantCulture)),
                  new XElement(Namespace + "changefreq", (options.ChangeFrequency ?? PageChangeFrequency.Daily).ToString().ToLowerInvariant()),
                  new XElement(Namespace + "priority", options.Priority ?? 0.5)
                  );