fix: remove highs log file default

[ollie-bell]

Closes # (if applicable).

Changes proposed in this Pull Request

The linopy interface to highs seems to be the only solver interface that will default to writing a log file if the users leaves log_fn as None. This may be intentional but it doesn't seem like it! It's only for the io_api="direct" route this happens.

I realised this because I am passing log_file in solver_options instead of using log_fn argument, but what I was specifying in solver_options was being ignored.

Checklist

• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.

[FabianHofmann]

awesome as always @ollie-bell. would you mind adding a release note?

[ollie-bell]

awesome as always @ollie-bell. would you mind adding a release note?

Done!