Issue #3246673 by phenaproxima, tedbow: Move PendingUpdatesValidator into Package Manager

automatic_updates.services.yml

package_manager/package_manager.services.yml

...e_manager/src/EventSubscriber/PendingUpdatesValidator.php

package_manager/tests/fixtures/db_update.php

...ager/tests/src/Kernel/ComposerExecutableValidatorTest.php

package_manager/tests/src/Kernel/DiskSpaceValidatorTest.php

...manager/tests/src/Kernel/PackageManagerKernelTestBase.php

..._manager/tests/src/Kernel/PendingUpdatesValidatorTest.php

tests/fixtures/post_update.php

...ReadinessValidation/PackageManagerReadinessChecksTest.php

...ernel/ReadinessValidation/PendingUpdatesValidatorTest.php