feat: Add FreeBSD 15 support

[wombelix]

Fixes #160

[wombelix]

FreeBSD 13.5: pkg: No packages available to install matching 'cpdup-freebsd' have been found in the repositories, caused by 7857f76, likely shouldn't be addressed anymore with 13.5 being EOL in 4 months from now.

FreeBSD 14.2:

Failed to start an instance: INVALID_ARGUMENT: Not Found 404 Not Found
POST [https://compute.googleapis.com:443/compute/v1/projects/cirrus-ci-community/zones/us-central1-c/instances](https://compute.googleapis.com/compute/v1/projects/cirrus-ci-community/zones/us-central1-c/instances)
{
"error": {
"code": 404,
"message": "The resource 'projects/freebsd-org-cloud-dev/global/images/family/freebsd-14-2' was not found",

So, the PR checks failing for unrelated reasons. I tested the code in this PR on a 15.0-RELEASE system and successfully installed the resulting image on a test vm.