fix(governance): replace multisig to operator

[ahramy]

AXE-4769

• replace multisig to operator in the service governance contract.
• Whether the address is a multisig is or not is not really relevant to the contract anymore.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.54%. Comparing base (b53530c) to head (283041a).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #185   +/-   ##
=======================================
  Coverage   98.54%   98.54%           
=======================================
  Files          48       48           
  Lines         758      758           
  Branches      157      157           
=======================================
  Hits          747      747           
  Misses          1        1           
  Partials       10       10           

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