mate.mate-desktop: 1.20.3 -> 1.21.0 (#41991)

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-desktop/versions.

These checks were done:

- built on NixOS
- Warning: no invocation of /nix/store/dx51na6smf7cpbav7zgb2x5xyyfbpcbp-mate-desktop-1.21.0/bin/mate-color-select had a zero exit code or showed the expected version
- /nix/store/dx51na6smf7cpbav7zgb2x5xyyfbpcbp-mate-desktop-1.21.0/bin/mate-about passed the binary check.
- Warning: no invocation of /nix/store/dx51na6smf7cpbav7zgb2x5xyyfbpcbp-mate-desktop-1.21.0/bin/.mate-color-select-wrapped had a zero exit code or showed the expected version
- /nix/store/dx51na6smf7cpbav7zgb2x5xyyfbpcbp-mate-desktop-1.21.0/bin/.mate-about-wrapped passed the binary check.
- 2 of 4 passed binary check by having a zero exit code.
- 0 of 4 passed binary check by having the new version present in output.
- found 1.21.0 with grep in /nix/store/dx51na6smf7cpbav7zgb2x5xyyfbpcbp-mate-desktop-1.21.0
- directory tree listing: https://gist.github.com/b54e2f1e430d08d273ba8dab30a7f562
- du listing: https://gist.github.com/74e6289b7a86dee591389adc61e024ed