Update adb-vnet-injection to use azurerm v4

[r3stl355]

One of several updates to existing templates to make them compatible with azurerm v4, tracked in #144

Changes:

1. Add subscription_id parameter: it is mandatory in azurerm v4
2. Remove no_public_ip: it is true by default in azurerm v4
3. Move the Teraform required module declaration to providers.tf for consistency with other modules
4. Update README.md

[r3stl355]

Is this what you had in mind by #144 @alexott? If so then I'll proceed with other templates, if not then let me know what I need to adjust

[alexott]

@r3stl355 yes - that's what I meant by that - make sure that all modules work with azurerm 4.x