[Backport 2.x] Ensure index templates are not applied to system indices

[opensearch-trigger-bot[bot]]

Backport 1e7c12232a963ad0409da73b0a921f1b727c607c from #16418.

[github-actions[bot]]

:white_check_mark: Gradle check result for e1a86fa6377dd9ab11e91cb9de877d02c92c3800: SUCCESS

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 25.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 71.85%. Comparing base (c26a49e) to head (e1a86fa). Report is 5 commits behind head on 2.x.

Files with missing lines	Patch %	Lines
...h/cluster/metadata/MetadataCreateIndexService.java	25.00%	2 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## 2.x #16518 +/- ## ============================================ + Coverage 71.83% 71.85% +0.02% - Complexity 65318 65379 +61 ============================================ Files 5312 5312 Lines 304985 304994 +9 Branches 44436 44437 +1 ============================================ + Hits 219079 219161 +82 + Misses 67609 67544 -65 + Partials 18297 18289 -8 ```

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