🔖 Bump version to 0.6.8

[marcelwa]

Description

This PR bumps the version to 0.6.8.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• I have added a changelog entry.
• I have created/adjusted the Python bindings for any new or updated functionality.
• I have made sure that all CI jobs on GitHub pass.
• The pull request introduces no new warnings and follows the project's style guidelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.15%. Comparing base (2ef6a23) to head (019d44b).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #645   +/-   ##
=======================================
  Coverage   98.14%   98.15%           
=======================================
  Files         245      245           
  Lines       37235    37235           
  Branches     1805     1804    -1     
=======================================
+ Hits        36544    36547    +3     
+ Misses        690      688    -2     
+ Partials        1        0    -1     

see 5 files with indirect coverage changes

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