Update version number to 9.1.0.rc1

CMake/vtkVersion.cmake

 # VTK version number components.
 set(VTK_MAJOR_VERSION 9)
 set(VTK_MINOR_VERSION 1)
-set(VTK_BUILD_VERSION 20211001)
+set(VTK_BUILD_VERSION 0)
 
 if (NOT VTK_MINOR_VERSION LESS 100)
   message(FATAL_ERROR