feat: support runner AuthenticationType configuration

* Update main.tf

add gitlab_runner_major_version to the config template to support AuthenticationType when version 15 or higher

* Update runner-config.tpl

add AuthenticationType = "IAM" only if version 15 or higher

Author: gravitybacklight

Diff for: main.tf

@@ -131,6 +131,7 @@ locals {
       bucket_name                       = local.bucket_name
       shared_cache                      = var.cache_shared
       sentry_dsn                        = var.sentry_dsn
+      gitlab_runner_major_version       = tonumber(split(".", var.gitlab_runner_version)[0])
     }
   )
 }

Diff for: template/runner-config.tpl

@@ -37,6 +37,8 @@ log_format = "json"
       BucketName = "${bucket_name}"
       BucketLocation = "${aws_region}"
       Insecure = false
+%{ if gitlab_runner_major_version >= 15 }      AuthenticationType = "IAM"
+%{ endif ~}
   [runners.machine]
     IdleCount = ${runners_idle_count}
     IdleTime = ${runners_idle_time}