Migrate GitHub URLs to `archlinux-downgrade`

[atreyasha]

Checklist

• Admin:
  • [x] No duplicate PRs
• Integration:
  • [x] Update unit tests
  • [x] Update shell completions
  • [x] Update configuration file
• Documentation:
  • [x] Update usage function
  • [x] Update mandoc
  • [x] Update readme
  • [x] Update locales
• Release:
  • [x] Update change log
  • [x] Update DOWNGRADE_VERSION

Description

This PR migrates existing GitHub URLs to archlinux-downgrade. A redirect from the old URL already exists, but just thought it would be prudent to do it anyways.

As a reminder (to myself), I would also have to update the source URL in the PKGBUILD before our next release.

[pbrisbin]

Unfortunately, "downgrade" as an Organization name was taken. I assume you're OK with what I chose instead? Now's the time to bikeshed :)

[atreyasha]

Unfortunately, "downgrade" as an Organization name was taken

Aww :/

I assume you're OK with what I chose instead? Now's the time to bikeshed :)

Yes this is still good