Update various packaging data

I initially just wanted to fix the Vcs-* issue as https://tracker.debian.org/pkg/wlgreet complained about it, but while I was at it, decided to also fix some other minor issues as well.

• d/control: Point Vcs-* fields to Salsa
• ci: Switch to using Salsa's CI recipe + disable 2 jobs
• d/copyright: Use Apache-2.0 license identifier instead of Apache-2
• lintian: Fix trailing whitespace issues
• d/control: Declare compliance with Policy 4.6.2
• d/patches: Add DEP-3 header
• d/watch: Update pattern to only find tags

Thanks for packaging wlgreet :-)