Try to move php-tuf/composer-stager to a dev dependency until package_manager is stable

So the tests are passing when we move

php-tuf/composer-stager

to a dev dependency.

However, the test in \Drupal\BuildTests\Composer\Template\ComposerProjectTemplatesTest::testMinimumStabilityStrictness uses composer --working-dir=$root info --format=json to get all the dependencies from

This list also contains dev-dependencies, so it will still fail if we bump testMinimumStabilityStrictness to stable.

andypost → closed merge request !10413