Export: Specify a policy range in exported files

This should reduce policy-related warnings coming from the
generated files, but in a way that doesn't increase the minimum
CMake version for consumers.

Relates: #20561

Source/cmExportFileGenerator.cxx

@@ -923,12 +923,14 @@ void cmExportFileGenerator::GeneratePolicyHeaderCode(std::ostream& os)
   /* clang-format on */
 
   // Isolate the file policy level.
-  // We use 2.6 here instead of the current version because newer
-  // versions of CMake should be able to export files imported by 2.6
-  // until the import format changes.
+  // Support CMake versions as far back as 2.6 but also support using NEW
+  // policy settings for up to CMake 3.17 (this upper limit may be reviewed
+  // and increased from time to time). This reduces the opportunity for CMake
+  // warnings when an older export file is later used with newer CMake
+  // versions.
   /* clang-format off */
   os << "cmake_policy(PUSH)\n"
-     << "cmake_policy(VERSION 2.6)\n";
+     << "cmake_policy(VERSION 2.6...3.17)\n";
   /* clang-format on */
 }