Use ED25519 ssh key

[jsf9k]

🗣 Description

This pull request modifies the AMI to use an ED25519 ssh key instead of an RSA ssh key.

💭 Motivation and context

Guacamole now supports non-RSA keys, and we changed cisagov/guacamole-packer to use an ED25519 key in cisagov/guacamole-packer#92.

🧪 Testing

I built a new AMI for our staging COOL and verified that it functions as expected with the new Guacamole AMI that was built as part of the testing for cisagov/guacamole-packer#92. Doing this also required the changes from cisagov/cool-assessment-terraform#216.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All new and existing tests pass.

✅ Pre-merge checklist

• [x] Revert dependencies to default branches.
• [x] Finalize version.

✅ Post-merge checklist

• [x] Create a release. Note that terraform-post-packer has not been run for this release since (per @dav3r) this AMI has not made it beyond initial testing.