Tidy up and fix ajax_example

Problem/Motivation

One test is failed. And got warnings of deprecation code and coding standards

........F..                                                       11 / 11 (100%)

Time: 2.1 minutes, Memory: 6.00 MB

There was 1 failure:

1) Drupal\Tests\ajax_example\FunctionalJavascript\EntityAutocompleteTest::testSubmitDriven
Failed asserting that actual size 0 matches expected size 2.

examples/ajax_example/tests/src/FunctionalJavascript/EntityAutocompleteTest.php:62



Remaining deprecation notices (11)

  11x: Drupal\Tests\BrowserTestBase::$defaultTheme is required in drupal:9.0.0 when using an install profile that does not set a default theme. See https://www.drupal.org/node/3083055, which includes recommendations on which theme to use.
    1x in AjaxExampleMenuTest::testAjaxExampleLinks from Drupal\Tests\ajax_example\Functional
    1x in DependentDropdownTest::testDependentDropdown from Drupal\Tests\ajax_example\Functional
    1x in DynamicFormSectionsTest::testDynamicFormSections from Drupal\Tests\ajax_example\Functional
    1x in AjaxWizardTest::testWizardSteps from Drupal\Tests\ajax_example\FunctionalJavascript
    1x in AjaxWizardTest::testWizardPreviousStepsValues from Drupal\Tests\ajax_example\FunctionalJavascript
    1x in AutotextfieldsTest::testAutotextfields from Drupal\Tests\ajax_example\FunctionalJavascript
    1x in DependentDropdownTest::testDependentDropdown from Drupal\Tests\ajax_example\FunctionalJavascript
    1x in DynamicFormSectionsTest::testDynamicFormSections from Drupal\Tests\ajax_example\FunctionalJavascript
    1x in EntityAutocompleteTest::testSubmitDriven from Drupal\Tests\ajax_example\FunctionalJavascript
    1x in SimplestTest::testAutotextfields from Drupal\Tests\ajax_example\FunctionalJavascript
    1x in SubmitDrivenTest::testSubmitDriven from Drupal\Tests\ajax_example\FunctionalJavascript


FILE: examples/ajax_example/src/Form/Wizard.php
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FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
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 85 | WARNING | t() calls should be avoided in classes, use \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead
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FILE: examples/ajax_example/src/Form/Autotextfields.php
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FOUND 0 ERRORS AND 1 WARNING AFFECTING 1 LINE
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 70 | WARNING | t() calls should be avoided in classes, use \Drupal\Core\StringTranslation\StringTranslationTrait and $this->t() instead
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