Increase required RTK version to 2.5 for accurate fixed forced detection

Commit ddc1885 accounts for a required
RTK change, RTKConsortium/RTK@a4ba257,
which has been released starting with RTK v2.5.

CMakeLists.txt

@@ -255,8 +255,8 @@ IF(GATE_USE_RTK OR GATE_USE_ITK)
     SET(ITK_MINIMAL_VERSION 4.12)
   ENDIF()
   FIND_PACKAGE(ITK ${ITK_MINIMAL_VERSION} REQUIRED COMPONENTS ${ITK_COMPONENTS})
-  IF(GATE_USE_RTK AND "${RTK_VERSION_MAJOR}.${RTK_VERSION_MINOR}" VERSION_LESS 2.0)
-    MESSAGE(FATAL_ERROR "RTK version 2.0 is required")
+  IF(GATE_USE_RTK AND "${RTK_VERSION_MAJOR}.${RTK_VERSION_MINOR}" VERSION_LESS 2.5)
+    MESSAGE(FATAL_ERROR "RTK version 2.5 is required")
   ENDIF()
   INCLUDE(${ITK_USE_FILE})
   STRING(REPLACE "-pedantic" "" CMAKE_CXX_FLAGS ${CMAKE_CXX_FLAGS})