change version specifiers to specify minimum versions

From https://www.terraform.io/docs/language/modules/develop/providers.html:

If you are writing a shared Terraform module, constrain only the minimum required provider version using a >= constraint. This should specify the minimum version containing the features your module relies on, and thus allow a user of your module to potentially select a newer provider version if other features are needed by other parts of their overall configuration.

We were previously using ~> for all specifiers which breaks consumers of the module who rely on, e.g., k8s provider resources only present in the 2.4 release.