Issue #2267545 by ParisLiakos: Standardize to RFC 5424 log levels.

core/includes/bootstrap.inc

core/includes/common.inc

core/includes/errors.inc

core/lib/Drupal/Core/Logger/LogLevelMapper.php

core/lib/Drupal/Core/Logger/LoggerChannel.php

core/lib/Drupal/Core/Logger/RfcLogLevel.php

core/lib/Drupal/Core/Logger/RfcLoggerTrait.php

core/modules/dblog/dblog.admin.inc

core/modules/dblog/src/Controller/DbLogController.php

core/modules/dblog/src/Logger/DbLog.php

core/modules/dblog/src/Tests/DbLogTest.php

core/modules/dblog/src/Tests/Views/ViewsIntegrationTest.php

core/modules/migrate/src/MigrateMessage.php

core/modules/syslog/src/Logger/SysLog.php

.../modules/system/src/Tests/Module/InstallUninstallTest.php

core/modules/system/src/Tests/Module/ModuleTestBase.php

...sts/Drupal/Tests/Core/Logger/LoggerChannelFactoryTest.php

core/tests/Drupal/Tests/Core/Logger/LoggerChannelTest.php