chore: bump version to 2.6.0

[jignaciopm]

Description

Bump the version to 2.6.0 so a release can be cut with the Verawood dependency support merged in #247.

Updates the version in the three version files:

• tutor/openedx_ai_extensions/__about__.py
• backend/openedx_ai_extensions/__init__.py
• frontend/package.json

and adds the CHANGELOG.rst entry.

[openedx-webhooks]

Thanks for the pull request, @jignaciopm!

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.42%. Comparing base (a3ec27a) to head (45245f1).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #249   +/-   ##
=======================================
  Coverage   95.42%   95.42%           
=======================================
  Files          71       71           
  Lines        8516     8516           
  Branches      451      451           
=======================================
  Hits         8126     8126           
  Misses        292      292           
  Partials       98       98           

Flag	Coverage Δ
unittests	95.42% <ø> (ø)

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[felipemontoya]

Thanks for the PR @jignaciopm. This was needed to run the release correctly