use cfnKeyPair instead of cdk-ec2-key-pair

[ahammond]

Per udondan/cdk-ec2-key-pair#53 (comment) the author of cdk-ec2-key-pair is deprecating the repo and suggests we start using the now-standard https://docs.aws.amazon.com/cdk/api/v2/docs/aws-cdk-lib.aws_ec2.CfnKeyPair.html

[udondan]

Not deprecating. 😸 See comment in the linked issue above.