Update guzzlehttp/psr7 to 2.1.0

The two fails are the two cases of ProviderRepositoryTest::testEmptyProviderList() (checking for the case where either an empty string or an empty array is returned as the JSON content of the provider database:

Drupal\Tests\media\Functional\ProviderRepositoryTest::testEmptyProviderList with data set "empty array" ('[]')
Failed asserting that exception of type "TypeError" matches expected exception "Drupal\media\OEmbed\ProviderException". Message was: "Double\GuzzleHttp\Psr7\Response\P1::getBody(): Return value must be of type Psr\Http\Message\StreamInterface, string returned" at
/var/www/html/core/modules/media/src/OEmbed/ProviderRepository.php:88
/var/www/html/core/modules/media/tests/src/Functional/ProviderRepositoryTest.php:39
/var/www/html/vendor/phpunit/phpunit/src/Framework/TestResult.php:722

The test method code:

    $response = $this->prophesize('\GuzzleHttp\Psr7\Response');
    $response->getBody()->willReturn($content);

    $client = $this->createMock('\GuzzleHttp\Client');
    $client->method('request')->withAnyParameters()->willReturn($response->reveal());
    $this->container->set('http_client', $client);

    $this->expectException(ProviderException::class);
    $this->expectExceptionMessage('Remote oEmbed providers database returned invalid or empty list.');
    $this->container->get('media.oembed.provider_repository')->getAll();

GuzzleHttp\Psr7\MessageTrait::getBody() is changed in the 2.x branch to have a return typehint of StreamInterface. (There was no return typehint in the 1.x branch.) This means that the expected exception can't ever be thrown because there's a TypeError before we get to that point.

It makes me suspect the test wasn't quite testing the correct thing to begin with -- even in 1.x, the docs say getBody() should only return StreamInterface or NULL. I suspect what we want to test is that the contents of the response evaluate to the empty values. I don't think we want to change the test to expect TypeError because the point is to test our custom exception handling.

Locally, #6 does the trick (when the correct namespace for Utils is added which I missed at first). Thanks @phenaproxima!

(Assuming that passes, I'll split it into its own issue since it's wrong-ish in HEAD as well.)

Forgot to mention here what I mentioned on the D10 roadmap. Let's split this into two issues:

1. Update the MR here for the latest HEAD, review, commit.
2. Open a new issue where we try to replace Diactoros PSR-17 services with guzzlehttp/psr7 ones.