mate.libmatemixer: 1.20.1 -> 1.21.0 (#43192)

Semi-automatic update generated by https://github.com/ryantm/nixpkgs-update tools.

This update was made based on information from https://repology.org/metapackage/libmatemixer/versions.

These checks were done:

- built on NixOS

- 0 of 0 passed binary check by having a zero exit code.
- 0 of 0 passed binary check by having the new version present in output.
- found 1.21.0 with grep in /nix/store/rf6swqf4zz4aqbis00vmga92jz30lnqz-libmatemixer-1.21.0
- directory tree listing: https://gist.github.com/26d3098e20c8910a026178b4deba670a
- du listing: https://gist.github.com/e928bc54962d0f4e091f69990262af22