Issue #3245428 by tedbow: Remove event names and use event classes instead.

src/AutomaticUpdatesEvents.php

src/Event/ExcludedPathsSubscriber.php

src/Event/PostCommitEvent.php

src/Event/PreCommitEvent.php

src/Event/PreStartEvent.php

src/Event/ReadinessCheckEvent.php

src/Event/UpdateEvent.php

src/Event/UpdateRefreshSubscriber.php

src/Updater.php

src/Validation/ReadinessValidationManager.php

src/Validator/ComposerExecutableValidator.php

src/Validator/CoreComposerValidator.php

src/Validator/DiskSpaceValidator.php

src/Validator/PendingUpdatesValidator.php

src/Validator/StagedProjectsValidator.php

src/Validator/UpdateVersionValidator.php

src/Validator/WritableFileSystemValidator.php

...omatic_updates_test/src/ReadinessChecker/TestChecker1.php

...matic_updates_test2/src/ReadinessChecker/TestChecker2.php

tests/src/Functional/UpdaterFormTest.php