Remove reliance on experimental feature

[hardbyte]

Description. What happened?

The Terraform configuration must be valid before initialization so that
Terraform can determine which modules and providers need to be installed.
╷
│ Error: Experiment has concluded
│ 
│   on .terraform/modules/network/_variables.tf line 2, in terraform:
│    2:   experiments = [module_variable_optional_attrs]
│ 
│ Experiment "module_variable_optional_attrs" is no longer available. The final feature corresponding to this experiment differs from the experimental form and is available in the Terraform language from
│ Terraform v1.3.0 onwards.

Terraform clearly stated that this experiment is subject to change, e.g. output from terraform init:

This feature is currently an opt-in experiment, subject to change in future releases based on feedback.

The migration guide - which includes requiring terraform 1.3.0 or newer is here.

Steps to reproduce

Use this module with a 1.3.x release of terraform.

Expected behavior

Module should work with latest stable release of terraform

Actual behavior

Experiment "module_variable_optional_attrs" is no longer available.

Occurrence

Frequently

Relevant log output

No response

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