v0.20.0: Release 0.20.0

New features:
- add basalt, harzburgite, impactor fields

Breaking changes:
- drop support for Python 3.8
- support numpy >= 2.0

Fixes:
- discovery of output files from outside of the run directory is now
  correct
- `Tcond` field variable is now correctly supported with both legacy and
  HDF5 output

Documentation:
- installation instructions use uv instead of pip

Internal:
- use uv and just instead of tox
- various github workflow improvements
- mkdocs always generate the list of config options
- drop `setuptools_scm`, version number is set manually