Start using #config_target in SitemapSettingsForm

Problem/Motivation

In 🌱 Drop support for D9, D10.0, D10.1, D10.2 Active , we dropped support for Drupal 10.1 and earlier.

We can now simplify the configuration form by making use of #config_target → in SitemapSettingsForm.

(note this issue is split from #3465601: Improve configuration schema and how we use it)

Proposed resolution

Convert the form controls to use #config_target where it is possible to do so.

As part of this, we should convert SitemapSettingsForm::isPathValid() into a configuration constraint (examples to come, sorry I don't have any off the top of my head).

Remaining tasks

1. Write a patch
2. Review and feedback
3. RTBC and feedback
4. Commit
5. Release

User interface changes

None.

API changes

None.

Data model changes

None.