Issue #3436671 by narendraR, Wim Leers, phenaproxima: Add validation constraints to system.cron

...tests/config_override_test/config/install/system.cron.yml

...tests/src/Functional/ConfigInstallProfileOverrideTest.php

core/modules/dblog/tests/src/Kernel/DbLogTest.php

core/modules/system/config/install/system.cron.yml

core/modules/system/config/schema/system.schema.yml

core/modules/system/system.post_update.php

...tem/tests/src/Functional/Update/CronLoggingUpdateTest.php

.../src/Kernel/Migrate/d6/MigrateSystemConfigurationTest.php

.../src/Kernel/Migrate/d7/MigrateSystemConfigurationTest.php

core/profiles/standard/config/install/system.cron.yml

...s/testing_config_overrides/config/install/system.cron.yml

...sts/Drupal/KernelTests/Core/Config/ConfigImporterTest.php

...sts/fixtures/config_install/testing_config_install.tar.gz