fix: remove upper case regex check rule for attribute codes respecting the docs

Author: drc0

app/code/Magento/Catalog/Model/Product/Attribute/Repository.php

@@ -219,8 +219,8 @@ public function getCustomAttributesMetadata($dataObjectClassName = null)
      */
     protected function generateCode($label)
     {
-        $code = substr(preg_replace('/[^a-zA-Z_0-9]/', '_', $this->filterManager->translitUrl($label)), 0, Attribute::ATTRIBUTE_CODE_MAX_LENGTH);
-        $validatorAttrCode = new \Zend_Validate_Regex(['pattern' => '/^[a-zA-Z][a-zA-Z_0-9]{0,29}[a-zA-Z0-9]$/']);
+        $code = substr(preg_replace('/[^a-z_0-9]/', '_', $this->filterManager->translitUrl($label)), 0, Attribute::ATTRIBUTE_CODE_MAX_LENGTH);
+        $validatorAttrCode = new \Zend_Validate_Regex(['pattern' => '/^[a-z][a-z_0-9]{0,29}[a-z0-9]$/']);
         if (!$validatorAttrCode->isValid($code)) {
             $code = 'attr_' . ($code ?: substr(md5(time()), 0, 8));
         }
@@ -236,7 +236,7 @@ protected function generateCode($label)
      */
     protected function validateCode($code)
     {
-        $validatorAttrCode = new \Zend_Validate_Regex(['pattern' => '/^[a-zA-Z][a-zA-Z_0-9]{0,' . Attribute::ATTRIBUTE_CODE_MAX_LENGTH . '}$/']);
+        $validatorAttrCode = new \Zend_Validate_Regex(['pattern' => '/^[a-z][a-z_0-9]{0,' . Attribute::ATTRIBUTE_CODE_MAX_LENGTH . '}$/']);
         if (!$validatorAttrCode->isValid($code)) {
             throw InputException::invalidFieldValue('attribute_code', $code);
         }