Fix MindOpt floating release

[PeterKlein11]

Changes proposed in this Pull Request

MindOpt's floating license token fails to release unless m.dispose() is called. Adding this under the MindOpt class in solvers.py fixes the issue. However, I am not sure if this is best place to dispose of the model or if the returned m value gets used further.

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.
• [ x] I consent to the release of this PR's code under the MIT license.