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

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 4eb4499..cfd1a4e 100644
--- a/CMakeLists.txt
+++ b/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}")