Issue #2902406 by mfernea: Fix 'Drupal.Classes.UseLeadingBackslash' coding standard

...upal/Component/HttpFoundation/SecuredRedirectResponse.php

...b/Drupal/Component/Plugin/ContextAwarePluginInterface.php

...pal/Component/Plugin/Exception/InvalidDecoratedMethod.php

core/lib/Drupal/Core/Config/ConfigBase.php

core/lib/Drupal/Core/Routing/RouteProvider.php

core/lib/Drupal/Core/StreamWrapper/TemporaryStream.php

core/modules/content_translation/content_translation.install

.../content_translation_test/content_translation_test.module

...s/help/tests/modules/help_page_test/help_page_test.module

core/modules/help/tests/modules/help_test/help_test.module

...ts/modules/more_help_page_test/more_help_page_test.module

core/modules/media/media.install

...nu_link_content/src/Plugin/migrate/process/d6/LinkUri.php

core/modules/rest/src/Annotation/RestResource.php

...dules/system/tests/modules/common_test/common_test.module

...ma_converter/entity_test_schema_converter.post_update.php

.../experimental_module_test/experimental_module_test.module

...s/system/tests/src/Kernel/Extension/ModuleHandlerTest.php

core/modules/taxonomy/tests/src/Functional/LegacyTest.php

core/phpcs.xml.dist