feat:use gitlab cluster agent instead of certificates

Description

Replaces the Gitlab cluster certificates with Gitlab Cluster agents, since certificates have been deprecated.

Supporting information

• gitlab-org/gitlab#327908 (closed)

Testing instructions

• In your respective grove-template fork, create a new Gitlab Cluster Agent token by going to the Infrastructure -> Kubernetes Clusters menu and tapping the "Connect a new cluster(agent)" button.
• Fill in the name for your agent and note the generated token on the Register screen.
• Add the CI/CD variable TF_VAR_gitlab_cluster_agent_token with the newly generated token.
• ./tf plan && ./tf apply or deploy using Gitlab.
• Once the deploy is complete, your agent's status will be "Connected"

Dependencies

• Staging repos for AWS and DO have been updated

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated

Additional context

• 14.10.0 is the most recent version on the helm chart. There is no v15 available so the configuration complains.