Require CMake 3.13 or higher, enable policies through 3.31

Also drop unused code for compatibility with CMake < 2.6.

assigned to @brad.king

Do: stage

mentioned in merge request !332 (merged)

Do: unstage

This is staged as part of !332 (merged), but should be merged first.

mentioned in merge request cmake/cmake!10517 (merged)

resolved all threads

Do: merge

mentioned in commit 3f2891c3

merged manually