cidrnetmask() produce an error with IPv6

[shinbunbun]

• Closes cidrnetmask() should error with IPv6 #30680

Make cidrnetmask() produce an error when trying to create a netmask for IPv6.

• ref: https://pkg.go.dev/net#IP.To4

[hashicorp-cla]

All committers have signed the CLA.

[shinbunbun]

@laurapacilio @kmoe

Hi, I have tried to solve this Issue.
If you don't mind, could you please review it?

[crw]

Thanks for the submission!

[kmoe]

Thanks for this. Looks good. Error message could have a bit more context - see comment.

[shinbunbun]

@kmoe
Thank you for your review!
I accepted your suggestion.

[github-actions]

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[github-actions]

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