Fatal errors - must be compatible with NodeRevisionController

Problem/Motivation

Currently this module is not compatible with diff version 2.1.1

PHP Fatal error: Declaration of Drupal\diff_moderate\Controller\NodeModerateRevisionController::revisionOverview(Drupal\node\NodeInterface $node) must be compatible with Drupal\diff\Controller\NodeRevisionController::revisionOverview(Drupal\node\NodeInterface $node): array in /var/www/html/docroot/modules/contrib/diff_moderate/src/Controller/NodeModerateRevisionController.php on line 25

PHP Fatal error: Declaration of Drupal\diff_moderate\Controller\NodeModerateRevisionController::compareNodeRevisions(Drupal\node\NodeInterface $node, $left_revision, $right_revision, $filter) must be compatible with Drupal\diff\Controller\NodeRevisionController::compareNodeRevisions(Drupal\node\NodeInterface $node, $left_revision, $right_revision, $filter): array in /var/www/html/docroot/modules/contrib/diff_moderate/src/Controller/NodeModerateRevisionController.php on line 45

Proposed resolution

Make the code compatible with the parent classes.

Remaining tasks

Merge patch and release new version.