Amazon-eks-ami: Make gp3 the default EBS volume type

[mmerkes]

What would you like to be added:

In December 2020, Amazon EBS announced gp3 volumes, the next generation of general purpose volumes. gp3 volumes are more performant, flexible and have lower costs than gp2 volumes, the current default, and they are available in all commercial and govcloud regions.

We should verify that there are no downsides to this and would not impact any existing customers, but seems like a reasonable change.

Changing it here would make it the default, and we should consider adding a new user variable to set the volume type to simplify it for customers who want to use a different volume type.

And probably update the CFN template.

Why is this needed:

gp3 volumes are the latest generation of EBS volumes and offer benefits over gp2 AMIs.

[mmerkes]

We've discussed this issue, and we want to do some additional validation and performance testing before we make gp3 volumes the default for AMIs. Once we are able to do that, we will add additional details here, including the results.