Release 0.12.2

Release 0.12.2

```
Colin Walters (8):
      Add 2021.3 feature
      Re-export glib, gio
      Deny unused results, warn on missing docs (except auto/)
      Add new GLib 0.14 variant types for metadata types
      Fix build with --features=v2021_3, use in CI by default
      Add more documentation for --features=v2021_3
      Use glib-sys via re-exported `glib::ffi` (and similar for gio)
      Release 0.12.2

Luca BRUNO (5):
      cargo: fix version in features chain
      lib: fix 'dox' feature
      cargo: point to docs.rs and clean up
      ostree: release 0.12.1
      cargo: bump ostree-sys to 0.8.1
```