refactor(upgrader): clean up the UpgraderInterface and simplify the contract

[cgorenflo]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 96.33%. Comparing base (6639369) to head (d917e2c).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
contracts/axelar-gateway/src/contract.rs	33.33%	4 Missing ⚠️
contracts/upgrader/src/contract.rs	84.61%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #84      +/-   ##
==========================================
- Coverage   96.64%   96.33%   -0.31%     
==========================================
  Files          36       37       +1     
  Lines        2267     2076     -191     
==========================================
- Hits         2191     2000     -191     
  Misses         76       76              

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