fix: update HiGHS URLs and naming

[FabianHofmann]

Changes proposed in this Pull Request

• Replace all dead maths.ed.ac.uk/hall/HiGHS/ links with https://highs.dev/
• Fix dead options link (HighsOptions.set) → https://ergo-code.github.io/HiGHS/stable/options/definitions/
• Use "HiGHS" consistently in docstrings instead of "Highs" (code identifiers unchanged)

Files changed: README.md, doc/index.rst, doc/prerequisites.rst, doc/release_notes.rst, linopy/solvers.py

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.