Allow upgrading Redis version

[markdboyd]

Background

The broker supports Redis versions 5.0.6 to 7.0. However, the broker does not support upgrading your engine version, so if you created an instance on an old version, you're stuck with that version.

This is especially problematic because there are new Redis features only available on newer Redis versions like logging: https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/Log_Delivery.html

Acceptance Criteria

• Update broker to allow customers to upgrade their Redis engine version
• Add unit tests for new behavior
• Manually test upgrade process
• Add acceptance test

[markdboyd]

This work was started on this branch: https://github.com/cloud-gov/aws-broker/compare/allow-redis-update?expand=1

[ccostino]

In addition to upgrading the engine version, would it also be possible to update the plan used in place too, similar to how that's supported in the RDS broker? Thanks!