There can be multiple problem here.
Firstly, it only works with EC2-VPC. It doesn't works with EC2-classic
[EC2-VPC only] Adds one or more egress rules to a security group for
use with a VPC. Specifically, this action permits instances to send
traffic to one or more destination CIDR IP address ranges, or to one
or more destination security groups for the same VPC. This action
doesn't apply to security groups for use in EC2-Classic. For more
information, see Security Groups for Your VPC in the Amazon Virtual
Private Cloud User Guide .
Secondly, boto3 does contains some documentation error. as mentioned by @Tayler, perhaps you should just omit
CidrIp. Honestly, I don't see why you need to create such egress rules, since it is default for all security group.
If you wish to remove all the rules in the group, you need to use revoke_security_group_egress. authorize_security_group_egress cannot "update" rules, it only add rules.