replace to_yaml() with stdlib::to_yaml()

sine puppetlabs/stdlib 9 to_yaml() is deprecated and stdlib::to_yaml()
is the successor. This isn't a breaking change because we already use
stdlib::to_yaml() in other places in this module.

manifests/webhook/config.pp

@@ -6,7 +6,7 @@
   file { 'webhook.yml':
     ensure  => $r10k::webhook::config_ensure,
     path    => $r10k::webhook::config_path,
-    content => to_yaml($r10k::webhook::config),
+    content => stdlib::to_yaml($r10k::webhook::config),
     notify  => Service['webhook-go'],
   }
 }