Implement a system check for deprecated mail settings

[Jacob Walls]

Because the deprecated mail settings will stop working in Django 7.0, this could lead to data loss if someone misses the deprecation warnings and fails to migrate to MAILERS.

For a similar scenario with STORAGES, on the forum we ​appeared to agree that in retrospect, we should have implemented a temporary system check in core.checks.compatibility despite some disagreement about what a more robust, permanent solution might look like for retiring settings.

An example where this was implemented before is in 354acd04af524ad82002b903df1189581c51cabe, although it's not a perfect analogue because that check only checked for conflicts between old & new settings--which we do have right now in 6.1, raising a runtime ImproperlyConfigured--not the presence of the old settings at all.

(Interestingly, the ImproperlyConfigured is swallowed by manage.py shell...)

Anyway, I think we should consider adding a system check during the deprecation period.

[Mike Edmunds]

Note DEP 0018 contemplates two similar ​MAILERS-related system checks as future work.

The "Missing "default" alias in MAILERS" check would catch this starting in 7.0 (because MAILERS defaults to {} starting in 7.0).

[Mike Edmunds]

[Jacob, this would be release blocker for 7.0, right? Not for 6.1.]