Improve variable names for QutritAmplitudeDamping channel

[Gabriel-Bottrill]

Context:
The QutritAmplitudeDamping channel #5503 and #5757 variable names are named with numbers, when making the TritFlip channel #5784 I realized that it would be better to specify subspaces for clarity. This PR is meant to change the name of QutritAmplitudeDamping's variables.

Description of the Change:
Change the name of the QutritAmplitudeDamping channel's variables from gamma_1, gamma_2, and gamma_3 to gamma_01, gamma_02, and gamma_12, respectively.

Benefits:
Makes the code more clear for which variables affect which subspace, reducing the need to check the docs.

Possible Drawbacks:
Changes the name of variables of development code.

Related GitHub Issues:
N/A

[Gabriel-Bottrill]

Hi @albi3ro, @glassnotes, @mudit2812, and @trbromley; @glassnotes pointed out that I should fix the names of QutritAmplitudeDamping gamma to specify the subspace, this PR does this.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.66%. Comparing base (7384db3) to head (7725ff5).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5799      +/-   ##
==========================================
- Coverage   99.66%   99.66%   -0.01%     
==========================================
  Files         414      414              
  Lines       39490    39194     -296     
==========================================
- Hits        39359    39061     -298     
- Misses        131      133       +2     

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