Clarify advice given by compatibility deprecation message for CMake < 3.10

Since commit 84db8506ff (Deprecate compatibility with CMake versions older
than 3.10, 2024-10-03, v3.31.0-rc1~23^2) the `cmake_minimum_required` and
`cmake_policy` commands warn if the project does not enable policies as
of 3.10 or higher.  Clarify the advice given in the warning message about
how to update the version specification, particularly without actually
requiring a newer minimum version of CMake.