Add 1.x release compatibilty

Problem/Motivation

We are currently working hard on releasing the sites module on drupal.org and had some serious refactoring done in terms of current_site handling and switching. Also a new settings API has been introduced and a QueryableSitesInterface which allows filtering views for example.

Those new mechanisms now allow us to generate single sitemaps which then can generate there data independently which should increase generation significantly and also allows adding single site sitemaps to queue for re-generation.

We also don't need to rely on a single sitemap configuration for all sites anymore. With the new site settings it should be possible to add bundle config on a per site basis now if needed. In combination with the sites_group module it could also be possible to provide default sitemap settings on a per group type basis.

Proposed resolution

Use the new API's to change how sitemaps are generated and configured.

Remaining tasks

- Commit and make a release candidate.
- Update existing MR's to work with the new code.

API changes

Sitemap configs are getting generated on a per site basis directly in the entity storage of simple_sitemaps. This way we don't produce config-diffs and can work with a content based site creation approach.

Data model changes

Configuration needs to be enabled for existing sites in order to have sitemaps.