vtkCriticalSection: actually deprecate

Mark it as deprecated in 9.1.0. However, because it never warned prior
to 9.2.0, it cannot actually be removed right now. Instead, remove it
when 9.2.0 APIs are removed (and add a marker to make it easy to find
it).

Common/Core/vtkCriticalSection.cxx

@@ -12,6 +12,12 @@
      PURPOSE.  See the above copyright notice for more information.
 
 =========================================================================*/
+
+// Hide VTK_DEPRECATED_IN_9_1_0() warnings for this class.
+// Remove with VTK_DEPRECATED_IN_9_2_0 because it was not actually deprecated
+// in 9.1.0.
+#define VTK_DEPRECATION_LEVEL 0
+
 #include "vtkCriticalSection.h"
 #include "vtkObjectFactory.h"
 

Common/Core/vtkCriticalSection.h

@@ -37,9 +37,13 @@
 #define vtkCriticalSection_h
 
 #include "vtkCommonCoreModule.h" // For export macro
+#include "vtkDeprecation.h"      // For VTK_DEPRECATED_IN_9_1_0
 #include "vtkObject.h"
 #include <mutex> // for std::mutex
 
+// Remove with VTK_DEPRECATED_IN_9_2_0 because it was not actually deprecated
+// in 9.1.0.
+VTK_DEPRECATED_IN_9_1_0("Use std::mutex instead")
 class VTKCOMMONCORE_EXPORT vtkCriticalSection : public vtkObject
 {
 public:

Documentation/release/dev/deprecate-vtkCriticalSection.md

+# vtkCriticalSection deprecation
+
+`vtkCriticalSection` was intended to be deprecated in VTK 9.1.0, but a warning
+was never added to it. VTK now has the warning present as it was originally
+intended.