Use GitHub tags page in debian/watch

This allows building packages after the tag was pushed but before the corresponding GitHub release was published.

The same change was already done in deb-icinga2: deb-icinga2@e8e9e552

requested review from @htriem

Pipeline status is pretty much irrelevant as master does not build from releases/tags anyways.

approved this merge request

enabled an automatic merge when the pipeline for 7c2c32a0 succeeds

mentioned in commit 3b69e60e

merged