🔥 drop Python 3.8 support

[burgholzer]

Description

Python 3.8 will reach end of live in October 2024.
Qiskit is already issuing deprecation warnings since a couple of releases when using it on Python 3.8.
This PR officially drops support for Python 3.8 in mqt-qcec.

Checklist:

• The pull request only contains commits that are related to it.
• I have added appropriate tests and documentation.
• 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 95.2%. Comparing base (4fbe57c) to head (05c000e).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #454     +/-   ##
=======================================
- Coverage   95.3%   95.2%   -0.1%     
=======================================
  Files         40      40             
  Lines       1836    1837      +1     
  Branches     224     224             
=======================================
  Hits        1750    1750             
- Misses        86      87      +1     

Flag	Coverage Δ
cpp	95.6% <ø> (-0.1%)	⬇️
python	93.7% <ø> (ø)

see 2 files with indirect coverage changes