FIX Class name in _t() call in installer and run text collector

lang/en.yml

@@ -1,6 +1,4 @@
 en:
-  SilverStripe\Admin\LeftAndMain:
-    VersionUnknown: unknown
   SilverStripe\AssetAdmin\Forms\UploadField:
     Dimensions: Dimensions
     EDIT: Edit

src/Dev/Install/Installer.php

@@ -150,7 +150,7 @@ PHP
             $adminMember = DefaultAdminService::singleton()
                 ->findOrCreateAdmin(
                     $username,
-                    _t(DefaultAdminService::class . '.DefaultAdminFirstname', 'Default Admin')
+                    _t('SilverStripe\\Security\\DefaultAdminService.DefaultAdminFirstname', 'Default Admin')
                 );
             $adminMember->Email = $username;
             $adminMember->Password = $password;