Bump CMake policy version to avoid deprecation warning (#1499)

Starting with CMake 3.27, there will be a warning for compat levels
below CMake 3.5.

Co-authored-by: Jordan Bayles <[email protected]>
diff --git a/jsoncppConfig.cmake.in b/jsoncppConfig.cmake.in
index 76570bc..fdd9fea 100644
--- a/jsoncppConfig.cmake.in
+++ b/jsoncppConfig.cmake.in
@@ -1,5 +1,5 @@
 cmake_policy(PUSH)
-cmake_policy(VERSION 3.0)
+cmake_policy(VERSION 3.0...3.26)
 
 @PACKAGE_INIT@