Replace drupal_set_message by the messenger service

Looks generally good, just one thing:

+++ b/src/Form/StockInventoryControlForm.php
@@ -5,10 +5,64 @@ namespace Drupal\commerce_simple_stock\Form;
+   * @param \Drupal\Core\Messenger\MessengerInterface $messenger
+   *   The Query service.
+   * @param \Drupal\Core\Entity\Query\QueryFactory $entityQuery
+   *   The Entity type manager service.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entityTypeManager
+   *   The messenger service.
+   */

I think the labels are mismatched here

Thanks @kkalaskar, feel free to implement the trait if you want, we can just add a dependency > 8.5 in the info file.

+++ b/src/Form/StockInventoryControlForm.php
@@ -154,10 +208,10 @@ class StockInventoryControlForm extends FormBase {
+          $this->messenger->addStatus($this->t($productVariation->getTitle() . ': ' . $productVariation->field_stock->value));

I don't think the t() call here is needed, it would translate twice

The current patch doesn't apply to the latest dev.

Thanks all!