Use #config_target for settings forms

Problem/Motivation

Drupal Core has a new API for form validation using #config_target, see https://www.drupal.org/node/3373502 →

Proposed resolution

Update configuration forms to use #config_target

Remaining tasks

1. Write a merge request
2. Review
3. Commit

User interface changes

None

API changes

None

Data model changes

None