Start using cross-object referencing for input variable validation

[ocofaigh]

In terraform 1.9, terraform added cross-object referencing for input variable validations: https://www.hashicorp.com/blog/terraform-1-9-enhances-input-variable-validations

Actions:

• Check if there is any code terraform code (modules, submodules, examples, or DAs) where we are using regex checks for cross variable validation, and if so update the code to use this new feature.
• Review the variables to see if it makes sense to add any cross variable validation.
• Bump the required terraform version to >= 1.9.0 anywhere in the repo we specify the required terraform version. We should do this even if we are not using the cross variable validation feature to future proof it incase we ever add it (or other tf 1.9 features) to the code.
• As reference, see POC https://github.com/terraform-ibm-modules/terraform-ibm-resource-group/pull/653/files