Update the Terraform examples in the Vault documentation to use auth_login_jwt instead

JWT auth now has first-class support since v3.9.0: https://github.com/hashicorp/terraform-provider-vault/releases/tag/v3.9.0

See https://ops.gitlab.net/gitlab-com/gl-infra/config-mgmt/-/merge_requests/4334

Part of gitlab-com/gl-infra&787 (closed)