[Terraform provider] Remove Unified Approval related code

Description

We need to remove the deprecated required_approval_count field related code from the terraform provider.

The affected files are:

• resource_gitlab_group_protected_environment.go
• resource_gitlab_project_protected_environment.go

and the respective spec files.

Open questions

We should probably add a documentation with migration path recommendations from the old configuration. Can we make a good error messages so users would know what is wrong in case they use legacy configuration?

How to test locally

TBA