Update RDS and EC2 instance classes to latest generation

[edkeeble]

Is your feature request related to a problem? Please describe.
Our RDS and EC2 instances are running on instance classes two generations behind the latest. Newer generations are both more powerful and less expensive to run. For example, our RDS instances are t2.micro. The latest generation instance is t4g.micro. Let's compare:

t2.micro

Core Count: 1
vCPU: 1
CPU Credits/hour: 6
Memory(GiB): 1
Network Performance (Gbps): Low to Moderate
Price per hour: $0.017

t4g.micro

vCPU: 2
Memory(GiB): 1
EBS Burst Bandwidth (Mbps): Up to 2,085
Network Performance (Gbps): Up to 5
Price per hour: $0.016

Our EC2 instances are t2.medium. The latest are t4g.medium.

t2.medium

On-demand Hourly Rate: $0.0464
vCPU: 2
Memory: 4GiB
Network performance: Low to Moderate

t4g.medium

On-demand Hourly Rate: $0.0336
vCPU: 2
Memory: 4GiB
Network performance: Up to 5 Gigabit

Describe the solution you'd like
We should upgrade these instances.

RDS Instances can either be upgraded in place or we can take a snapshot and restore it into a new instance. Downtime isn't a huge deal for these services, since the user-facing component (CASEI) is static and only hits the backend at deploy time.

EC2 instances can be upgraded following this guide: https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/ec2-instance-resize.html#change-instance-type-of-ebs-backed-instance