Use ClientInterface in SecurityAdvisoriesFetcher constructor

Problem/Motivation

In unit tests that mock the guzzle client interface, the SecurityAdvisoriesFetcher throws an error like

TypeError: Argument 3 passed to Drupal\system\SecurityAdvisories\SecurityAdvisoriesFetcher::__construct() must be an instance of GuzzleHttp\Client, instance of Mock_ClientInterface_f0228a1b given

Steps to reproduce

Create a Kernel test that sets the http_client service to a mocked object using a GuzzleHttp\ClientInterface::class. For example,

    $guzzle = $this->createMock(ClientInterface::class);
    $guzzle->method('request')->will($this->returnCallback([
      $this,
      'guzzleCallback',
    ]));
    \Drupal::getContainer()->set('http_client', $guzzle);
    Role::create(['label' => "Foo", 'id' => "foo"])->save();

Proposed resolution

Simply replace the constructor type hint to use ClientInterface instead of Client

Remaining tasks

Replace the Type Hint

User interface changes

None

API changes

None

Data model changes

None

Release notes snippet