GuzzleHttp\Utils::defaultUserAgent() is deprecated Since guzzlehttp/guzzle 8.5

Problem/Motivation

There are many failures in the "Updated dependencies" CI job because of guzzle deprecation.
For example: https://git.drupalcode.org/project/drupal/-/jobs/13614258

Workspace Form Persistence (Drupal\Tests\workspaces\Kernel\WorkspaceFormPersistence)
     ✘ Form persistence
       ┐
       ├ TypeError: Symfony\Component\Routing\Attribute\Route::__construct(): Argument #16 ($alias) must be of type Symfony\Component\Routing\Attribute\DeprecatedAlias|array|string, null given, called in /builds/project/drupal/core/lib/Drupal/Core/Routing/Attribute/Route.php on line 111
       │
       │ /builds/project/drupal/vendor/symfony/routing/Attribute/Route.php:67
       │ /builds/project/drupal/core/lib/Drupal/Core/Routing/Attribute/Route.php:111
       │ /builds/project/drupal/core/modules/system/src/Controller/ThemeController.php:55
       │ /builds/project/drupal/core/lib/Drupal/Core/Routing/AttributeRouteDiscovery.php:371
       │ /builds/project/drupal/core/lib/Drupal/Core/Routing/AttributeRouteDiscovery.php:101
       │ /builds/project/drupal/core/lib/Drupal/Core/Routing/AttributeRouteDiscovery.php:67
       │ /builds/project/drupal/core/lib/Drupal/Core/Routing/StaticRouteDiscoveryBase.php:93
       │ /builds/project/drupal/vendor/symfony/event-dispatcher/EventDispatcher.php:254
       │ /builds/project/drupal/vendor/symfony/event-dispatcher/EventDispatcher.php:206
       │ /builds/project/drupal/vendor/symfony/event-dispatcher/EventDispatcher.php:56
       │ /builds/project/drupal/core/lib/Drupal/Core/Routing/RouteBuilder.php:116
       │ /builds/project/drupal/core/lib/Drupal/Core/ProxyClass/Routing/RouteBuilder.php:83
       │ /builds/project/drupal/core/tests/Drupal/KernelTests/RouteProvider.php:36
       │ /builds/project/drupal/core/tests/Drupal/KernelTests/RouteProvider.php:53
       │ /builds/project/drupal/core/lib/Drupal/Core/Routing/UrlGenerator.php:413
       │ /builds/project/drupal/core/lib/Drupal/Core/Routing/UrlGenerator.php:248
       │ /builds/project/drupal/core/lib/Drupal/Core/Render/MetadataBubblingUrlGenerator.php:92
       │ /builds/project/drupal/core/lib/Drupal/Core/Url.php:762
       │ /builds/project/drupal/core/lib/Drupal/Core/Render/Element/RenderElementBase.php:387
       │ /builds/project/drupal/core/lib/Drupal/Core/Render/Element/RenderElementBase.php:239
       │ /builds/project/drupal/core/lib/Drupal/Core/Form/FormBuilder.php:1074
       │ /builds/project/drupal/core/lib/Drupal/Core/Form/FormBuilder.php:1137
       │ /builds/project/drupal/core/lib/Drupal/Core/Form/FormBuilder.php:606
       │ /builds/project/drupal/core/lib/Drupal/Core/Form/FormBuilder.php:347
       │ /builds/project/drupal/core/modules/workspaces/tests/src/Kernel/WorkspaceFormPersistenceTest.php:77
       ┴
    
    1 test triggered 1 deprecation:
    
    1) /builds/project/drupal/vendor/symfony/deprecation-contracts/function.php:25
    Since guzzlehttp/guzzle 8.5: GuzzleHttp\Utils::defaultUserAgent() is deprecated and will be removed in 9.0. Use GuzzleHttp\ClientInterface::MAJOR_VERSION to build the string instead.
    
    Triggered by:
    
    * Drupal\Tests\workspaces\Kernel\WorkspaceFormPersistenceTest::testFormPersistence
      /builds/project/drupal/core/modules/workspaces/tests/src/Kernel/WorkspaceFormPersistenceTest.php:72

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet