app-backup/backup2l: version bump 1.6, migration to GitHub
Version 1.6 has already been released back in 2016 but on GitHub instead of SourceForge: https://github.com/gkiefer/backup2l/tags
The project has officially moved to GitHub for all releases 1.5 and later. Update ebuild for 1.5 to fetch from GitHub instead, then bump to version 1.6.