Adds lowercase validation rule translation (#6028)

* Adds lowercase validation rule translation

* Update validation.php

Co-authored-by: Dries Vints <dries@vints.io>

lang/en/validation.php

@@ -71,6 +71,7 @@ return [
     'ipv4' => 'The :attribute must be a valid IPv4 address.',
     'ipv6' => 'The :attribute must be a valid IPv6 address.',
     'json' => 'The :attribute must be a valid JSON string.',
+    'lowercase' => 'The :attribute must be lowercase.',
     'lt' => [
         'array' => 'The :attribute must have less than :value items.',
         'file' => 'The :attribute must be less than :value kilobytes.',