🧱 include MQT Core via FetchContent instead of as a submodule

[burgholzer]

Description

https://github.com/cda-tum/mqt-core was included as a submodule so far. This PR removes this submodule and uses the existing fetch content commands to include this dependency.

Furthermore, it adds a dependabot-like update workflow for the FetchContent-managed mqt-core dependency.

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 96.3%. Comparing base (015d0e6) to head (c693bcd).
Report is 53 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #407     +/-   ##
=======================================
- Coverage   96.3%   96.3%   -0.1%     
=======================================
  Files         35      35             
  Lines       1818    1817      -1     
  Branches     224     223      -1     
=======================================
- Hits        1751    1750      -1     
  Misses        67      67             

Flag	Coverage Δ
cpp	96.0% <ø> (-0.1%)	⬇️
python	97.2% <ø> (ø)

see 1 file with indirect coverage changes

[burgholzer]

The failure here will be resolved by #405