cidrnetmask() should error with IPv6

[laurapacilio]

Our documentation says that cidrnetmask() only works with IPv4. But a user tried using it with can(cidrnetmask()) and found out that it doesn't actually error with IPv6. For either IPv4 or IPv6, it returns a netmask.

It is technically invalid to create a netmask for IPv6, so we should correct the behavior of this function to return an error as promised.

Terraform Version

Terraform v1.1.7

Expected Behavior

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

Actual Behavior

A user found that for either IPv4 or IPv6, cidrnetmask() returns a netmask.

References

PR where user found this issue and provides additional context: #30660

[github-actions]

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