abi-check does not know which source is older, newer

Right before the release API version (LIBINTERFACE, LIBREVISION, LIBAGE) for older and newer libraries tends to be the same. Given that, commit hash can't be the determining factor here, Unix time of the commit should suit us better and is placed after the API version. The commit hash is preserved as it's useful to see it in the actual report.

See https://gitlab.isc.org/isc-projects/bind9/-/jobs/821031 for ABI check reports.

Fixes #1751 (closed).