Build the Nessus AMI on a Graviton (AMD64) instance

[jsf9k]

🗣 Description

This pull request modifies the Packer code to build the Nessus AMI on a Graviton (AMD64) instance. Note that this requires moving the AMI from Debian to Ubuntu and updating the version of Nessus that gets installed.

💭 Motivation and context

Running the AMI on Graviton should be cheaper and better for the environment.

🧪 Testing

All automated tests pass. I also built a staging AMI with these changes and verified that it functions as expected.

✅ Pre-approval checklist

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

✅ Pre-merge checklist

• Finalize version.

✅ Post-merge checklist

• Create a release.

[jsf9k]

Nessus does not support ARM64 on Debian. Newer versions of Nessus support ARM64 on Ubuntu, but not the old version that we are currently using.