Packer builder example: use impersonation instead of SA keys

[mikouaj]

The goal of this PR is to make example Packer build to use service account impersonation for Packer service account instead of using service account keys. This brings numerous advantages:

• makes solution more secure by eliminating keys in favour of IAM based control
• simplifies overall solution (no need to store keys in Secrets Manager, no need to decrypt them)

Additional changes made in this PR:

• upgrade default version of a Packer to 1.7.5
• migrated example Packer build configuration file from JSON format to HCL2 format. As of v1.7.0 this is preferred way of writing Packer configurations.
• updated example README file accordingly

[mikouaj]

@alygerharter @prabenzom can you take a look on this please ? :)