Migrate packer AMI definition to HCL2

[MatthiasScholz]

HCL2 is the successor for the current json definition and provides better tooling like fmtsupport.

• https://www.hashicorp.com/blog/announcing-hashicorp-packer-1-5-with-hcl2-support
• https://www.packer.io/guides/hcl