[Backport 7.x] Only sign core plugins and ignore min artifacts

[opensearch-trigger-bot[bot]]

Backport e6ec7d1782d25ee19bb1738dc943fc55d4fb1b2d from #564.

[codecov[bot]]

Codecov Report

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

Project coverage is 84.75%. Comparing base (9139375) to head (2f5c43d). Report is 2 commits behind head on 7.x.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 7.x #565 +/- ## ========================================= Coverage 84.75% 84.75% Complexity 114 114 ========================================= Files 117 117 Lines 682 682 Branches 75 75 ========================================= Hits 578 578 Misses 30 30 Partials 74 74 ```

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