Diagnostic API cleanups:

*IMPROVEMENT: Drupal's status report now only shows warning and error diagnostics.
*APIBREAK: Removed DiagnosticCheckInterface::getHookRequirementsArray() and DiagnosticsServiceInterface::getHookRequirementsArray().
*NEW: $floor parameter for filtering DiagnosticsServiceInterface::getRequirementsArray().
*NEW: $prefix_title parameter for hook_requirements() use of DiagnosticsServiceInterface::getRequirementsArray() output.