eliminate remaining go mod dependencies on argo cd and gitops engine

[krancour]

Fixes #956

This has been a long time coming. This PR removes the last of our Go mod dependencies on Argo CD and GitOps Engine.

With it, we also say goodbye to a litany of transitive dependencies.

This should make it considerably easier to upgrade the modules we do care about.

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[codecov]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (369fa29) 48.37% compared to head (efb7185) 48.37%.
Report is 3 commits behind head on main.

Files	Patch %	Lines
internal/credentials/credentials.go	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1235   +/-   ##
=======================================
  Coverage   48.37%   48.37%           
=======================================
  Files         127      127           
  Lines        9632     9632           
=======================================
  Hits         4659     4659           
  Misses       4812     4812           
  Partials      161      161           

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