Sitemap Plugin Versions Save

Some CI issues left to solve, but at least the plugin can be installed when upgrading to Sylius v1.12.

After a long time of alpha and beta testing, tagging v2.0.0. I get regularly questions about installation of the version mentioned in the README, hopefully this increases the useability.

Further improvements will be either done inside v2 via patch or minor releases, or in an upcoming major v3 release (i.e. split big sitemaps).

First beta release, finally :-)

Thanks @vicla31 for the fix! When having multiple channels, it could lead to duplicate URLs for multiple channels in multiple sitemaps.

Thanks to @vicla31 for the major support!