Issue #3087463 by heddn, ressa: Leverage update.xml to discover core security updates

automatic_updates.install

automatic_updates.module

src/Form/SettingsForm.php

src/ProjectInfoTrait.php

src/ReadinessChecker/ModifiedFiles.php

src/ReadinessChecker/ReadinessCheckerManager.php

src/ReadinessChecker/ReadinessCheckerManagerInterface.php

src/Services/InPlaceUpdate.php

src/Services/ModifiedFiles.php

tests/src/Build/InPlaceUpdateTest.php

tests/src/Kernel/ReadinessChecker/MissingProjectInfoTest.php

tests/src/Kernel/ReadinessChecker/ReadinessCheckerTest.php