As of version 4.0.0, attr_encrypted supports Rails 7.

This change relaxes the version requirement for attr_encrypted so that
there's a path to upgrade to Rails 7 while staying in the
`devise-two-factor` 4.x series, for apps who are ready for the Rails
update but not yet ready for the `devise-two-factor` 5.x update.

Relevant attr_encrypted version: attr-encrypted/attr_encrypted@b565876
Rails 7 changes: attr-encrypted/attr_encrypted#434

Runs spec assertions conditionally based on which version of
attr_encrypted is loaded.