mate.mate-applets: 1.20.2 -> 1.21.0 (#43176)

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

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

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/6q9mwpgw42jld019cnn6ka513c8zw1sc-mate-applets-1.21.0/bin/mate-cpufreq-selector had a zero exit code or showed the expected version
- Warning: no invocation of /nix/store/6q9mwpgw42jld019cnn6ka513c8zw1sc-mate-applets-1.21.0/bin/.mate-cpufreq-selector-wrapped had a zero exit code or showed the expected version
- 0 of 2 passed binary check by having a zero exit code.
- 0 of 2 passed binary check by having the new version present in output.
- found 1.21.0 with grep in /nix/store/6q9mwpgw42jld019cnn6ka513c8zw1sc-mate-applets-1.21.0
- directory tree listing: https://gist.github.com/a0ab2cac21cf6cae44f410df2b5c6284
- du listing: https://gist.github.com/0c9f7376cb17ad1d5e2a1bce03520edc