Write script to upgrade brokered databases services from gp2 to gp3

[markdboyd]

In order to optimize costs, we should update EBS storage from gp2 to gp3 volumes across all of our brokered plans

Acceptance Criteria

• A script is written and used to update all existing brokered RDS instances from gp2 to gp3

---

Security considerations

There are no meaningful security differences from gp2 to gp3 volumes, as far as I know, just cost and performance optimizations.

https://aws.amazon.com/ebs/general-purpose/#:~:text=gp3%20offers%20SSD%2Dperformance%20at,improving%20performance%20and%20reducing%20costs.

[markdboyd]

Closing in favor of #333