Add rocky-linux-cloud to GCE public projects

[t0m]

Add rocky-linux-cloud family to the public GCE projects array.

Closes #67

?       github.com/hashicorp/packer-plugin-googlecompute    [no test files]
ok      github.com/hashicorp/packer-plugin-googlecompute/builder/googlecompute  12.064s
ok      github.com/hashicorp/packer-plugin-googlecompute/post-processor/googlecompute-export    0.656s
ok      github.com/hashicorp/packer-plugin-googlecompute/post-processor/googlecompute-import    0.426s
?       github.com/hashicorp/packer-plugin-googlecompute/version    [no test files]

[hashicorp-cla]

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