fix mitigation matrix

[Edoardo-Pedicillo]

It closes #615.
I decided to delete create_annotated_heatmap since it is deprecated.
The suggested replacement is imshow from plotly express, but because of a problem with dash, it is not able to diplay the text on the matrix cells.
Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.
• Compatibility with Qibo modules (Please edit this section if the current pull request is not compatible with the following branches).
  • Qibo: master
  • Qibolab: main
  • Qibolab_platforms_qrc: main

[codecov]

Codecov Report

Merging #622 (624d4c0) into main (478360a) will decrease coverage by 0.01%.
Report is 63 commits behind head on main.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #622      +/-   ##
==========================================
- Coverage   95.90%   95.89%   -0.01%     
==========================================
  Files         102      102              
  Lines        7103     7097       -6     
==========================================
- Hits         6812     6806       -6     
  Misses        291      291              

Flag	Coverage Δ
unittests	95.89% <100.00%> (-0.01%)	⬇️

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Files	Coverage Δ
...cols/characterization/readout_mitigation_matrix.py	95.91% <100.00%> (-0.24%)	⬇️