clients: don't use deprecated fontconfig aliases

The "sans" and "mono" aliases for "sans-serif" and "monospace" are
deprecated[1].  Let's standardize on the non-deprecated versions, which were
already in use in some places.

[1]: https://gitlab.freedesktop.org/fontconfig/fontconfig/-/blob/be453bd1590b545d490aa024ef267948419b4d3a/fonts.conf.in#L33-67



Signed-off-by: Alyssa Ross <hi@alyssa.is>