ruby 3 support (doc update)

[tbugfinder]

The top readme of the project refers to use of ruby <2.8.

kitchen-terraform/README.md

Lines 51 to 55 in aab3c14

	Kitchen-Terraform aims to support all versions of Ruby that are in
	["normal" or "security" maintenance][ruby-branches], which is currently
	the interval of `>= 2.4, < 2.8`.
	[rbenv] can be used to manage versions of Ruby on the system.

Code itself refers to ruby 3, though.
https://github.com/newcontext-oss/kitchen-terraform/blob/master/kitchen-terraform.gemspec#L47