[questionable] fix kas config : assign same value for authentication_secret_file of gitlab and private_api

Could it be same?
(omnibus-gitlab assigns so)

Author: kkimurak

assets/runtime/config/gitlab-agent/gitlab-kas_config.yaml

@@ -5,7 +5,7 @@
 
 gitlab:
   address: http://localhost:8080{{GITLAB_RELATIVE_URL_ROOT}} # required
-  authentication_secret_file: {{GITLAB_KAS_SECRET}} # required
+  authentication_secret_file: {{GITLAB_AGENT_KAS_API_LISTEN_AUTHENTICATION_SECRET_FILE}} # required
   # ca_certificate_file: /server-ca.pem
   api_rate_limit:
     bucket_size: 250