ITK nightly version update

This commit updates:
  (1) SHA used in CMakeLists.txt to checkout ITK sources
          (InsightSoftwareConsortium/ITK@21cf6485e6)
  (2) VERSION variable set in itkVersion.py

It was automatically generated by the script update_python_version.sh [1]

[1] https://github.com/InsightSoftwareConsortium/ITKPythonPackage/blob/d1b6706/scripts/update_python_version.py

Author: kwrobot

CMakeLists.txt

@@ -74,8 +74,8 @@ if(ITKPythonPackage_SUPERBUILD)
 
   set(ITK_REPOSITORY "https://github.com/InsightSoftwareConsortium/ITK.git")
 
-  # ITK nightly-master 2020-12-21
-  set(ITK_GIT_TAG "8f12adfab3")
+  # ITK nightly-master 2020-12-22
+  set(ITK_GIT_TAG "21cf6485e6")
 
   #-----------------------------------------------------------------------------
   # A separate project is used to download ITK, so that it can reused

itkVersion.py

@@ -1,4 +1,4 @@
-VERSION = '5.1.2.dev20201221+775.g8f12adfab3'
+VERSION = '5.1.2.dev20201222+781.g21cf6485e6'
 
 def get_versions():
     """Returns versions for the ITK Python package.