Ruff config deprecation warning(s)

[qstokkink]

When running ruff, we get the following warning:

warning: The top-level linter settings are deprecated in favour of their counterparts in the `lint` section. Please update the following options in `.ruff.toml`:
  - 'dummy-variable-rgx' -> 'lint.dummy-variable-rgx'
  - 'fixable' -> 'lint.fixable'
  - 'ignore' -> 'lint.ignore'
  - 'select' -> 'lint.select'
  - 'unfixable' -> 'lint.unfixable'
  - 'pylint' -> 'lint.pylint'

We should update these entries to be in the lint section.