Bugfix/gurobi license and docs

[RobbieKiwi]

Changes proposed in this Pull Request

I noticed that the notebook in the docs is still failing even after the last fix where I added highs to the doc dependencies.

The problem

The init of gurobi in solvers.py was disabled in this commit when building the list of available solvers. This makes sense for performance but it seems to have led to the example notebooks breaking.
As there is no check for the gurobi license when building the available solvers list, the first solver in the list (gurobi) may not be usable if the licensing is not correct.
Currently the example notebooks are failing.

Proposed solutions

I propose to explicitly select "highs" in the example notebooks so that they always work without relying on a valid gurobi license.

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.