Fix backend call

[ElStabilini]

change from GlobalBackends() call to get_backend() call for compatibility with qibo v0.2.13

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

[alecandido]

@ElStabilini maybe you want to change once more the pyproject.toml and rerun poetry lock again to lock v0.2.13 (btw: since this PR, you will be only compatible with Qibo v0.2.13 or greater, so the correct specification is ^0.2.13).

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.99%. Comparing base (c280398) to head (51ebc1e).
Report is 3 commits behind head on 0.2.

Additional details and impacted files

@@            Coverage Diff             @@
##              0.2    #1050      +/-   ##
==========================================
- Coverage   97.02%   96.99%   -0.03%     
==========================================
  Files          98       98              
  Lines        7893     7893              
==========================================
- Hits         7658     7656       -2     
- Misses        235      237       +2     

Flag	Coverage Δ
unittests	96.99% <100.00%> (-0.03%)	⬇️

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

Files with missing lines	Coverage Δ
...qibocal/protocols/randomized_benchmarking/utils.py	94.38% <100.00%> (ø)
src/qibocal/protocols/readout_mitigation_matrix.py	100.00% <100.00%> (ø)
src/qibocal/protocols/state_tomography.py	98.64% <100.00%> (ø)
...rc/qibocal/protocols/two_qubit_state_tomography.py	99.28% <100.00%> (ø)

... and 1 file with indirect coverage changes

[alecandido]

@ElStabilini since this is doubly approved (and not even pointing to main...) feel free to merge it whenever you wish :)

[scarrazza]

In view of #1051, I believe we should port these changes in main too.