Enforce case-sensitive uniqueness for Cloud::Ec2::Configuration.external_id

In Rails 6.0.x, we used to have this deprecation warning:

Uniqueness validator will no longer enforce case sensitive comparison in
Rails 6.1. To continue case sensitive comparison on the :external_id
attribute in Cloud::Ec2::Configuration model, pass `case_sensitive:
true` option explicitly to the uniqueness validator.

All other instances of uniqueness validator already have either
`case_sensitive: true` or `case_sensitive: false`, so nothing had to be
done for those models.