🔥 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-core.

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 87.0%. Comparing base (11f7ab0) to head (77394c7).
Report is 103 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #684     +/-   ##
=======================================
- Coverage   87.1%   87.0%   -0.1%     
=======================================
  Files        125     125             
  Lines      13782   13781      -1     
  Branches    2164    2164             
=======================================
- Hits       12008   11995     -13     
- Misses      1774    1786     +12     

Flag	Coverage Δ
cpp	86.6% <ø> (-0.1%)	⬇️
python	99.7% <100.0%> (ø)

Files with missing lines	Coverage Δ
src/mqt/core/plugins/qiskit.py	100.0% <100.0%> (ø)

... and 2 files with indirect coverage changes