CI: Ruff linter settings warning

[lgolston]

Rationale

Running ruff check gives 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 `pyproject.toml`:
  - 'select' -> 'lint.select'
  - 'isort' -> 'lint.isort'

Implications

pyproject.toml is modified accordingly to avoid the warning.