🔧 add `merge_group` to workflow configs

[burgholzer]

Description

This tiny PR adds the merge_group label to the GitHub workflows in order to make them work with merge queues.

Checklist:

• [x] The pull request only contains commits that are related to it.
• [x] I have added appropriate tests and documentation.
• [x] I have made sure that all CI jobs on GitHub pass.
• [x] The pull request introduces no new warnings and follows the project's style guidelines.

[codecov[bot]]

Codecov Report

Merging #242 (77fd536) into main (335bcee) will decrease coverage by 0.2%. The diff coverage is n/a.

@@           Coverage Diff           @@
##            main    #242     +/-   ##
=======================================
- Coverage   96.2%   96.1%   -0.2%     
=======================================
  Files         34      34             
  Lines       1757    1757             
  Branches     221     221             
=======================================
- Hits        1691    1689      -2     
- Misses        66      68      +2     

Flag	Coverage Δ
cpp	95.7% <ø> (-0.1%)	:arrow_down:
python	97.3% <ø> (-0.3%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/checker/dd/DDEquivalenceChecker.cpp	90.8% <0.0%> (-1.0%)	:arrow_down:
mqt/qcec/compilation_flow_profiles.py	98.0% <0.0%> (-0.7%)	:arrow_down:

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