Fix CMake deprecation warning for compatibility with CMake < 3.10 (#1659)

* Fix uninitialized CMake variable in version.in

CMake's project() command sets jsoncpp_VERSION (lowercase prefix),
not JSONCPP_VERSION. The wrong variable caused the generated version
file to be empty.

Fixes #1656

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* Fix CMake deprecation warning for compatibility with CMake < 3.10

Bump JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION from 3.8.0 to 3.10.0
to silence the CMake 3.31 deprecation warning.

Fixes #1598

---------

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: baylesj

CMakeLists.txt

@@ -12,7 +12,7 @@
 # CMake versions greater than the JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION policies will
 # continue to generate policy warnings "CMake Warning (dev)...Policy CMP0XXX is not set:"
 #
-set(JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION "3.8.0")
+set(JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION "3.10.0")
 set(JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION "3.13.2")
 cmake_minimum_required(VERSION ${JSONCPP_OLDEST_VALIDATED_POLICIES_VERSION})
 if("${CMAKE_VERSION}" VERSION_LESS "${JSONCPP_NEWEST_VALIDATED_POLICIES_VERSION}")