Resolve "[Feature flag] Rollout of `allow_dots_on_tf_state_names`"

What does this MR do and why?

This feature flag was used to conditionally allow dots in Terraform state names. This FF has been enabled on GitLab.com since its existance and was only relevant for self-managed.

Now, we are removing it to always allow dots in Terraform state names.

Closes #385597 (closed)

How to set up and validate locally

1. Create a new project.
2. Create a pipeline with the Terraform.latest.gitlab-ci.yml template.
3. Set the TF_STATE_NAME variable to terraform.tfstate:

include:
  - template: Terraform.latest.gitlab-ci.yml

variables:
  TF_STATE_NAME: terraform.tfstate

4. Add main.tf with dummy content (e.g. single output definition of literal) and an http backend config:

terraform {
  backend "http" {}
}

output "foobar" {
  value = 42
}

5. Verify that pipeline runs and a TF state called terraform.tfstate exists.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #385597 (closed)