Issue #3191900: rrssb-plus library location in Drupal 9 environment

rrssb.install

@@ -28,7 +28,7 @@ function rrssb_requirements($phase) {
     $requirements['rrssb']['title'] = t('RRSSB+ library');
     $library = \Drupal::service('library.discovery')->getLibraryByName('rrssb', 'main');
     $downloadMessage =
-       t('Please download the RRSSB library using "drush rrssb-plugin" or from <a href="@link">@link</a>.', ['@link' => RRSSB_LIBRARY_URI]);
+       t('RRSSB library not found! On Drupal 9 and above, use composer to install the library. On Drupal 8, please download the RRSSB library using "drush rrssb-plugin" or from <a href="@link">@link</a>.', ['@link' => RRSSB_LIBRARY_URI]);
 
     if (!isset($library['version'])) {
       $requirements['rrssb']['value'] = t('Not installed');