[Backport 2.12] Fixes base64 encoded hardcoded admin credentials

[opensearch-trigger-bot[bot]]

Backport 74dfcd169d202c1b3eab8e17f3b7bff9461b85d6 from #1331

[codecov[bot]]

Codecov Report

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

Comparison is base (72c504f) 76.04% compared to head (fb0a64d) 76.48%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 2.12 #1333 +/- ## ============================================ + Coverage 76.04% 76.48% +0.43% - Complexity 1044 1045 +1 ============================================ Files 141 141 Lines 4784 4784 Branches 526 526 ============================================ + Hits 3638 3659 +21 + Misses 793 774 -19 + Partials 353 351 -2 ```

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