ci: Prepare release of version 2.1.0

[fsbraun]

Description

This PR updates the classifiers, changelog and removes the outdated direct django-treebeard dependency.

Related resources

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Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (d358846) 100.00% compared to head (b31eae5) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #72   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          128       128           
  Branches         7         7           
=========================================
  Hits           128       128           

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