release prep 1.6.0

[atravitz]

Make the PR:

• Create a new release-prep branch corresponding to the version name, e.g. release-prep-v1.2.0. Note: please follow semantic versioning.
• Check that all user-relevant updates are included in the news/ rever .rst files. You can backfill any additional items by making a new .rst, e.g. backfill.rst
• Run rever, e.g. rever 1.2.0. This will auto-commit docs/CHANGELOG.md and remove the .rst files from news/.
• Verify thatdocs/CHANGELOG.md looks correct.
• Make the PR and verify that CI/CD passes.
• Merge the PR into main.
• Make a PR into the example notebooks repository to update the version used in showcase/openfe_showcase.ipynb and .binder/environment.yml

After Merging the PR follow this guide

[github-actions]

No API break detected ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.14%. Comparing base (b8147d5) to head (332f54b).
⚠️ Report is 96 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1447      +/-   ##
==========================================
- Coverage   93.83%   93.14%   -0.70%     
==========================================
  Files         165      165              
  Lines       12433    12433              
==========================================
- Hits        11667    11581      -86     
- Misses        766      852      +86     

Flag	Coverage Δ
fast-tests	93.14% <ø> (?)
slow-tests	?

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