perf: update ec2 and rd2 instance types

bastion: t3a.micro is the latest x86_64 based ec2 instance available. It's marginally cheaper, faster, includes extra vCPU, and has better network performance. I would recommend t4g but it does not have a STIG AMI available.

database/mysql: db.t3.small was picked because it's faster, cheaper, and keeps the same size as the previous instance. db.t4g.small is not available, only medium and larger sizes.

database/postgres: db.t4g.medium is the newest ec2 instance type available in the burstable tier. It should be roughly the same performance, possibly better but it's cheaper.

server & agent: m6id.size was selected because it is the newest generation and offers a performance increase, and slightly better networking. It costs marginally more but it includes more instance storage, the EBS-backed version is the same price as m5.size but offers more performance.

Here's some links for pricing and additional information regarding m6i instance type.

https://aws.amazon.com/ec2/instance-types/m6i/
https://aws.amazon.com/ec2/pricing/on-demand/

Prices listed for AWS GovCloud West

instance.type	ondemand price	vCPUs	RAM	Instance Storage	Network Performance
m6id.4xlarge	$1.2016	16	64 GiB	1 x 950 SSD	Up to 12500 Megabit
m5d.4xlarge	$1.144	16	64 GiB	2 x 300 SSD	Up to 10 Gigabit

instance.type	ondemand price	vCPUs	RAM	Instance Storage	Network Performance
m6i.4xlarge	$0.968	16	64 GiB	EBS Only	Up to 12500 Megabit
m5.4xlarge	$0.968	16	64 GiB	EBS Only	Up to 10 Gigabit