Fix CS8981: Type names use only lower-cased ascii characters (may become reserved)

[poppastring]

The following type names use only lower-cased ascii characters, which may become reserved for the language:

• changefreq (changefreq.cs, GoogleSiteMap.cs)
• urlset (GoogleSiteMap.cs, urlset.cs)
• url (GoogleSiteMap.cs, url.cs)

Consider renaming these types to avoid future compatibility issues.

Files:

• DasBlog.Services\Seo\GoogleSiteMap\changefreq.cs
• DasBlog.Services\Seo\GoogleSiteMap\GoogleSiteMap.cs
• DasBlog.Services\Seo\GoogleSiteMap\url.cs
• DasBlog.Services\Seo\GoogleSiteMap\urlset.cs