🧱 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 92.5%. Comparing base (a80fd38) to head (4d51a06).
Report is 63 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #388     +/-   ##
=======================================
- Coverage   92.5%   92.5%   -0.1%     
=======================================
  Files         37      37             
  Lines       2561    2560      -1     
  Branches     309     309             
=======================================
- Hits        2371    2369      -2     
- Misses       190     191      +1     

Flag	Coverage Δ
cpp	94.5% <ø> (-0.1%)	⬇️
python	87.3% <ø> (ø)

see 2 files with indirect coverage changes