Allow enabling Stack with Spacelift Terraform provider

Workaround

Changing the configuration in the code and then doing the same change manually in the UI after the Terraform apply fails.

Problem

We have built a self-service portal in our repository for setting up Stacks. The whole configuration goes through review and is then automatically deployed by Spacelift. Sometimes we want to disable certain Stacks (when there is a problem) and then enable them back (when the issue is fixed). However, the spacelift_stack_activator resource does not allow the later transition and fails with: │ Error: could not enable stack foo: incompatible issuer