mate.mate-system-monitor: 1.20.1 -> 1.21.0 (#43187)

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-system-monitor/versions.

These checks were done:

- built on NixOS
- /nix/store/r2kdrj4kv3ljxdv8l9m6k5ihpij628s5-mate-system-monitor-1.21.0/bin/mate-system-monitor passed the binary check.
- /nix/store/r2kdrj4kv3ljxdv8l9m6k5ihpij628s5-mate-system-monitor-1.21.0/bin/.mate-system-monitor-wrapped passed the binary check.
- 2 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/r2kdrj4kv3ljxdv8l9m6k5ihpij628s5-mate-system-monitor-1.21.0
- directory tree listing: https://gist.github.com/f4f7bd0ff2528925fb7906a6dba7a165
- du listing: https://gist.github.com/a529d5cc92ba18328a47ad40d4a87b2a