Update QVM demo notebooks to use willow_pink

[pavoljuhas]

The following quantum virtual machine notebooks were updated to use
the new willow_pink processor by default:

• quantum_virtual_machine.ipynb
• qvm_basic_example.ipynb
• qvm_builder_code.ipynb
• qvm_stabilizer_example.ipynb

Small fixups in other reviewed notebooks.

Related to b/395705720

[pavoljuhas]

NB: can be also reviewed per-commit.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.69%. Comparing base (4c9b173) to head (778127e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #7428   +/-   ##
=======================================
  Coverage   98.69%   98.69%           
=======================================
  Files        1112     1112           
  Lines       97965    97965           
=======================================
+ Hits        96683    96686    +3     
+ Misses       1282     1279    -3     

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[eliottrosenberg]

Thanks, Pavol! General comment for all of these notebooks: let's always make "willow_pink" the default and maybe not even list the others since they are very old and people probably shouldn't be using them.

[eliottrosenberg]

Let's make "willow_pink" the default and maybe not even mention the others since they are very old and don't represent current performance.