Conflicts with Wordpress 5.5 Sitemaps

[rodruiz]

Because Wordpress 5.5 is using the query var "sitemap" to generate the "wp-sitemap.xml" file there is a conflict with this plugin. Both are using the same query var.

The quickest way to fix this is to use a different query var, something like "sitemap_msm".

I can submit a PR.

Thanks.