Change AutomaticUpdatesKernelTestBase::setReleaseMetadata() to return metadata for different projects based on request path

Problem/Motivation

in #3274830: Make ProjectInfo work with legacy version numbers we added \Drupal\Tests\automatic_updates\Kernel\ProjectInfoTest::setReleaseMetadataForProjects() to ensure that requests for the update XML, which return our test XML files, does not rely on request order but rather the path of the request, which changes per project.

Proposed resolution

we should just change `\Drupal\Tests\automatic_updates\Kernel\AutomaticUpdatesKernelTestBase::setReleaseMetadata` to do this and get rid of \Drupal\Tests\automatic_updates\Kernel\ProjectInfoTest::setReleaseMetadataForProjects()

So, to clarify, right now we do stuff like this in kernel tests:

$this->setReleaseMetadata([__DIR__ . '/../../fixtures/release-history/drupal.9.8.2.xml']);

What would be better is to do what \Drupal\Tests\automatic_updates\Kernel\ProjectInfoTest::setReleaseMetadataForProjects() does, where we specifically say "for this project, return this release metadata". So, I'd like to see something like this:

$this->setReleaseMetadata([
  'drupal' => __DIR__ . '/../../fixtures/release-history/drupal.9.8.2.xml',
]);

Remaining tasks

1. Move the approach taken by ProjectInfoTest::setReleaseMetadataForProjects() into AutomaticUpdatesKernelTestBase::setReleaseMetadata(), and update its doc comment to reflect the way it now works.
2. Change ProjectInfoTest to use setReleaseMetadata(), and remove setReleaseMetadataForProjects().
3. Change all the calls to setReleaseMetadata() in our kernel tests to use the new format.