⏫ Forwardport of #12737 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/12737.patch (created by @zamoroka) based on commit(s):
  1. 5cf0d34
  2. 7418f1a

Fixed GitHub Issues in 2.3-develop branch:
  - #11953: Product configuration creator does not warn about invalid SKUs (reported by @JanisE)

app/code/Magento/ConfigurableProduct/Ui/DataProvider/Product/Form/Modifier/ConfigurablePanel.php

@@ -5,6 +5,7 @@
  */
 namespace Magento\ConfigurableProduct\Ui\DataProvider\Product\Form\Modifier;
 
+use Magento\Catalog\Model\Product\Attribute\Backend\Sku;
 use Magento\Catalog\Ui\DataProvider\Product\Form\Modifier\AbstractModifier;
 use Magento\Ui\Component\Container;
 use Magento\Ui\Component\Form;
@@ -466,7 +467,17 @@ protected function getRows()
                         [],
                         ['dataScope' => 'product_link']
                     ),
-                    'sku_container' => $this->getColumn('sku', __('SKU')),
+                    'sku_container' => $this->getColumn(
+                        'sku',
+                        __('SKU'),
+                        [
+                            'validation' =>
+                                [
+                                    'required-entry' => true,
+                                    'max_text_length' => Sku::SKU_MAX_LENGTH,
+                                ]
+                        ]
+                    ),
                     'price_container' => $this->getColumn(
                         'price',
                         __('Price'),