Remove system.mail from SchemaCheckTrait ignored property paths

The pull request I just opened, is to check which tests fail when we remove this from Check Trait, so this is

Postponing this on actually landing the first issues, but all we needed to do to get this to work is install the config schema of the system module.

Thanks for getting this sample follow-up issue going!

There are 176 commits in this MR, but that's only because this is demonstrating how this will work after 📌 [PP-1] Configuration schema & required keys Needs review has landed.

The pattern:

1. Remove some lines from the list of ignored property paths: https://git.drupalcode.org/project/drupal/-/merge_requests/5209/diffs?co...
2. Result: test failures!
3. Next: make tests pass: https://git.drupalcode.org/project/drupal/-/merge_requests/5209/diffs?co...

Blockers are in! 😄