Enable flag group_authorized_agents by default

What does this MR do and why?

The feature flag being enabled by default makes it possible to share access to a Kubernetes Agent's CI/CD tunnel with projects other than the Kubernetes Agent's configuration project. By default, only the Kubernetes Agent's configuration project has CI/CD access.

Screenshots or screen recordings

KUBECONFIG injection works by default:

How to set up and validate locally

1. GDK needs a runner attached, and the gitlab-k8s-agent enabled

2. Delete any residual group_authorized_agents configuration:

   Feature.remove(:group_authorized_agents)

3. Verify that ::Feature.enabled?(:group_authorized_agents, default_enabled: :yaml) is true

4. Add an agent with configuration project gitlab-agent-testing/config with the following configuration:

   # config/.gitlab/agents/first/config.yaml
   ci_access:
     projects:
       - id: gitlab-agent-testing/ci-project # <-- We will run CI/CD in here
         access_as:
           agent: {}

5. In a separate project, gitlab-agent-testing/ci-project, add the following .gitlab-ci.yml:

   # .gitlab-ci.yml
   test:
     image: registry.gitlab.com/gitlab-org/cluster-integration/helm-install-image/releases/3.6.3-kube-1.17.17-alpine-3.14
     script:
       - kubectl config get-contexts

6. View the job logs for the test job

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.