Update minimum cmake from 3.7 to remove warnings

[derekbruening]

On my machine with cmake 3.31 I now get this warning (this is quite recent; my cmake may have been update recently):

CMake Deprecation Warning at CMakeLists.txt:49 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.

The warning is repeated a dozen times for all our other files.

We may as well update the minimum. I'm thinking of 3.16 which is the Ubuntu 20.04 version.