Migrating to Codeberg

The reasons for this change seem to be described in [0] with the following summary:

  > The contribution workflow in Guix has been facing several challenges:
  > difficult onboarding, lack of legibility, complex, unreliable, and
  > labor-intensive infrastructure, and lack of automation.  All these lead
  > to an experience that contributors often find frustrating and hinders
  > quality assurance efforts.  We propose to address these limitations by
  > migrating repositories, issue tracking, and patch tracking to Codeberg,
  > an integrated forge with a Web interface that is hosted by a non-profit.

They were still on savannah only. OMG.