lib/gitlab/ci/templates/Packer.gitlab-ci.yml: parse new *.pkr.hcl too

What does this MR do and why?

Parse the new *.pkr.hcl packer template files too

Since 1.7, HCL2 is the preferred way to write Packer configuration. See https://www.packer.io/docs/templates/hcl_templates

Screenshots or screen recordings

• N/A

How to set up and validate locally

• N/A

MR acceptance checklist

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