Remove support for terraform 0.12

[mskrajnowski]

Removing support for terraform 0.12 would let us use new features introduced in 0.13
https://github.com/hashicorp/terraform/blob/v0.13/CHANGELOG.md#0130-august-10-2020

This would let us drop create inputs entirely as it can be replaced with count or for_each on module level. This would let us greatly simplify the codebase which is covered in var.create checks.