2021.12

There's a notable new feature in this release from our Outreachy intern, which
is that override replace learned experimental support for fetching from
rpm-md repos. This makes it a bit easier to find newer (or older) packages
from repos.
PR: #3092

A notable bugfix is that rpm-ostree rebase now accepts ostree:// as a
prefix again, which fixes gnome-software:
PR: #3157

Other fixes include kernel overrides with newer depmod, and a docs tweak.

Thanks to all contributors!

Colin Walters (6):
      rust: Use `opt-level=1` for default `dev`
      extensions: Add support for per-extension repos and modules
      main: Remove pointless `async { ... }` + `.await` pair
      main: Remove lots of `Some` matching
      rebase: Parse `ostree://` prefix again
      Release 2021.12

Jonathan Lebon (2):
      libpriv/kernel: Handle new modules.builtin.alias.bin depmod file
      libpriv/kernel: Point to depmod source of truth

Rafael G. Ruiz (1):
      override replace: add experimental options

Zhangyuan Nie (1):
      docs: fix hyperlinks in architecture-daemon.md

dependabot[bot] (10):
      build(deps): bump tokio from 1.11.0 to 1.12.0
      build(deps): bump subprocess from 0.2.7 to 0.2.8
      build(deps): bump libc from 0.2.102 to 0.2.103
      build(deps): bump cxx from 1.0.54 to 1.0.55
      build(deps): bump tracing from 0.1.27 to 0.1.28
      build(deps): bump serde_json from 1.0.67 to 1.0.68
      build(deps): bump system-deps from 4.0.0 to 5.0.0
      build(deps): bump cxx-build from 1.0.54 to 1.0.55
      build(deps): bump nix from 0.22.1 to 0.23.0
      build(deps): bump curl from 0.4.38 to 0.4.39

Git-EVTag-v0-SHA512: dd71cae4fab04ce0b8e1ab495afb2d3b5ed6620510bf71ce1eb7666d1b50cc873e53da4f1174905985e0b989c7f88159ca82cb9a441f730d04ca98e449a8441b