diff -u b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
--- b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
+++ b/core/modules/image/src/Plugin/ImageEffect/ConvertImageEffect.php
@@ -63,7 +63,7 @@
     $extensions = \Drupal::service('image.toolkit.manager')->getDefaultToolkit()->getSupportedExtensions();
     $options = array_combine(
       $extensions,
-      array_map(['\Drupal\Component\Utility\Unicode', 'strtoupper'], $extensions)
+      array_map('mb_strtoupper', $extensions)
     );
     $form['extension'] = [
       '#type' => 'select',
only in patch2:
unchanged:
--- a/core/modules/migrate/tests/src/Unit/process/CallbackTest.php
+++ b/core/modules/migrate/tests/src/Unit/process/CallbackTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\Tests\migrate\Unit\process;
 
+use Drupal\Component\Utility\Unicode;
 use Drupal\migrate\Plugin\migrate\process\Callback;
 
 /**
@@ -37,9 +38,9 @@ public function testCallbackWithFunction() {
    * Test callback with a class method as callable.
    */
   public function testCallbackWithClassMethod() {
-    $this->plugin->setCallable(['\Drupal\Component\Utility\Unicode', 'strtolower']);
-    $value = $this->plugin->transform('FooBar', $this->migrateExecutable, $this->row, 'destinationproperty');
-    $this->assertSame('foobar', $value);
+    $this->plugin->setCallable([Unicode::class, 'ucfirst']);
+    $value = $this->plugin->transform('foobar', $this->migrateExecutable, $this->row, 'destinationproperty');
+    $this->assertSame('Foobar', $value);
   }
 
 }
only in patch2:
unchanged:
--- a/core/modules/node/tests/src/Kernel/SummaryLengthTest.php
+++ b/core/modules/node/tests/src/Kernel/SummaryLengthTest.php
@@ -2,7 +2,6 @@
 
 namespace Drupal\Tests\node\Kernel;
 
-use Drupal\Component\Utility\Unicode;
 use Drupal\Core\Datetime\Entity\DateFormat;
 use Drupal\KernelTests\KernelTestBase;
 use Drupal\node\Entity\Node;
@@ -72,9 +71,6 @@ protected function setUp() {
       'label' => 'Fallback',
       'pattern' => 'Y-m-d',
     ])->save();
-
-    // Enable multibyte support.
-    Unicode::setStatus(Unicode::STATUS_MULTIBYTE);
   }
 
   /**