docs: authorise kargo to argocd-update

[fykaa]

closes https://github.com/akuity/kargo/issues/2440

https://github.com/akuity/kargo/pull/2921#discussion_r1844558574

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Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 50.90%. Comparing base (34c6128) to head (c462695). Report is 9 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #2962 +/- ## ========================================== - Coverage 50.92% 50.90% -0.02% ========================================== Files 279 279 Lines 25180 25178 -2 ========================================== - Hits 12823 12818 -5 - Misses 11666 11669 +3 Partials 691 691 ```

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Successfully created backport PR for release-1.0:

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