refactor(module/virtual_network_gateway): Refactor module for VNG

[sebastianczech]

Description

PR delivers changes for VNG:

• README.md adjusted to new style with additional header file
• minimum Terraform version set to 1.3
• refactor variables:
  • descriptions
  • types with optionals
  • default values
  • validations
• simplify main.tf by removing not required try
• remove name generation logic from the module

Motivation and Context

307 , #333

How Has This Been Tested?

I checked that module using example included in PR.

Types of changes

• New feature (non-breaking change which adds functionality)

Checklist

• [ ] I have updated the documentation accordingly.
• [x] I have read the CONTRIBUTING document.
• [ ] I have added tests to cover my changes if appropriate.
• [x] All new and existing tests passed.

[acelebanski]

Hi Sebastian, shouldn't you make name a mandatory property everywhere in the module and get rid of prefix/suffix etc.? In order to move the entire name generation logic to example?

[sebastianczech]

@acelebanski thank you for feedback. I removed name_prefix and name_suffix. All resources have name attribute.