Update AWS disk type to gp3

Closes https://gitlab.com/gitlab-org/quality/gitlab-environment-toolkit/-/issues/156

Switched the default AWS type from gp2 to gp3. gp2 is now a burst based ssd which will lead to performance issues, gp3 is now the preferred and is also cheaper (AWS doesn't allow for any non SSD types for the root disk so we just default to the main one).

IOPS for Gitaly node was also misconfigured here, being too low in gp2. AWS is doing something different here where you can just configure the required IOPS directly now with gp3 instances so this is also now being done for Gitaly nodes by default.

Luckily AWS can handle the in place adjustment of disk types without data loss so this change should have no material impact.