Improve configuration schema and how we use it

Okay, I’ve decided to create a merge request for the “safest” change, i.e.: the one that only makes changes that maintain compatibility with Drupal 9.0, i.e.: changing certain types from string to path.

Merging the string → path changes.

Moving back to "Active" for the rest of the proposed changes

Note that part 1 of this change was released in sitemap-8.x-2.0-beta7 → ; but there are still outstanding parts of this issue to be released in the future.

From my own comment in #3465577-18: Split sitemap_book config schema from sitemap schema → :

I'm guessing that a lot of the nullable keys were the result of all plugin settings being lumped into a single array. You had to do that so one plugin could leave all the other settings as empty.

While this config improvement is going on, any unnecessary nullable properties should be removed.