[Housekeeping]: Validate `AccessList.name` with `validate_slug` (replace custom regex)

[pheus]

Proposed Changes

• Keep AccessList.name as a CharField.
• Add Django’s validate_slug validator to the field (no field-type change).
• Update tests accordingly.

Justification

Replace the custom RegexValidator for AccessList.name with Django’s built-in validate_slug to reduce custom code and resolve the test suite’s invalid escape sequence warning.

• Remove bespoke validation logic in favor of a built-in validator.
• Keep behavior explicit and maintainable.

References

• Related PR: Use slug validation for AccessList name #270