fix: Make discount code ID read only

[iamareebjamal]

Fixes #4529

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[codecov]

Codecov Report

Merging #4531 into development will not change coverage.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           development    #4531   +/-   ##
============================================
  Coverage        22.25%   22.25%           
============================================
  Files              470      470           
  Lines             4961     4961           
  Branches             6        6           
============================================
  Hits              1104     1104           
  Misses            3856     3856           
  Partials             1        1           

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