Update example Jupyter notebooks.

[vtjeng]

• Switch from Gurobi to HiGHS to enable repro without having a Gurobi license.
• Remove L2-norm example as HiGHS does not appear to support MIQP

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.68%. Comparing base (a1d5d06) to head (4331567). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #161 +/- ## ======================================= Coverage 91.68% 91.68% ======================================= Files 25 25 Lines 818 818 ======================================= Hits 750 750 Misses 68 68 ```

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