COMP: Bump itkeigen cmake_minimum to 3.16.3 and clarify dead-block wrap

Two changes in Modules/ThirdParty/Eigen3/src/itkeigen/CMakeLists.txt
addressing reviewer feedback:

* Bump cmake_minimum_required (line 2) from 3.10.2 to 3.16.3 to match
  the version ITK proper requires; the previous 3.10.2 was the older
  Eigen-import default and has no reason to differ from the rest of
  ITK's tree.

* Replace the if(FALSE) ... endif() wrapper used to disable the
  upstream Eigen CMakeLists with CMake's multiline-comment markers
  #[[ ... #]] and explicit "ITK - START/STOP DISABLE UPSTREAM EIGEN
  LOGIC" labels. Functionally identical (both leave the wrapped
  region inert), but IDEs and code-folding visualizers recognize
  CMake multiline comments and dim the dead region; if(FALSE) leaves
  the region looking active in most editors. The line 9
  cmake_minimum_required(VERSION 3.10.0) inside the wrapped region
  becomes a comment after this change, matching its prior dead state.

USE_SYSTEM_EIGEN compatibility for Eigen 3.3+ is preserved: this
file is only configured under USE_SYSTEM_EIGEN=OFF; consumers using
the system Eigen go through find_package(Eigen3 3.3 REQUIRED) and
never read this file.

Author: hjmjohnson

Modules/ThirdParty/Eigen3/src/itkeigen/CMakeLists.txt

@@ -1,10 +1,10 @@
 # Same version of ITK cmake_minimum_required
-cmake_minimum_required(VERSION 3.10.2)
+cmake_minimum_required(VERSION 3.16.3)
 
 project(Eigen3)
 
 # ITK doesn't compile anything here, just generates targets for the INTERFACE library.
-if(FALSE)
+#[[ # ITK - START DISABLE UPSTREAM EIGEN LOGIC
 
 cmake_minimum_required(VERSION 3.10.0)
 
@@ -828,7 +828,7 @@ message(STATUS "")
 message(STATUS "Configured Eigen ${EIGEN_VERSION_STRING}")
 message(STATUS "")
 
-endif() # Regular CMakeLists of Eigen ends here
+#]] # ITK -STOP DISABLE UPSTREAM EIGEN LOGIC
 
 ###############################################################################
 ################################ ITK ##########################################