scripts: new maintainer script validate-minimum-dependency-versions

Automate what we can.

This script could be added later to an encompassing script
'prepare-for-release' (which would also do other release-related steps).

The trick '2> >(tee "$log" >&2)' sends the error output to the console while
at the same time writing to a file. It uses bash-specific syntax (process
substitution: >(...) )
See https://stackoverflow.com/a/692407/2941347 .

Includes contributions by Mads Kiilerich.