dh_installchangelogs: Add --since option to include only recent entries
Changelogs of older and heavily developed packages can reach sizes of hundreds of kilobytes. Even after compression, such big changelogs are a waste of space and bandwidth worldwide.
The --since option trims changelogs to include only recent changelog
entries, for example all changelog entries added after a Debian release
(e.g. --since oldstable, --since debian10) or after a specific date
(e.g. --since 2020-04-18).
An additional commit enables --since oldstable by default in compat 14.