mate.libmatekbd: 1.20.2 -> 1.21.0 (#43193)

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

This update was made based on information from https://repology.org/metapackage/libmatekbd/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/gff3905ialfwgrpwyrhgaz56k1qzsphp-libmatekbd-1.21.0
- directory tree listing: https://gist.github.com/a9d8fc76f7c7e6f7b94c3c70dbf27d18
- du listing: https://gist.github.com/57420f2d15d064e6e84c8b2b3c6feaa1