fix(charts): update helm release thanos to v15.7.21

[renovate[bot]]

This PR contains the following updates:

Package	Update	Change
thanos (source)	patch	15.7.20 -> 15.7.21

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Release Notes

bitnami/charts (thanos)

### [`v15.7.21`](https://togithub.com/bitnami/charts/blob/HEAD/bitnami/thanos/CHANGELOG.md#15721-2024-08-19) - \[bitnami/thanos] Update index cache config for sharded store gateway ([#​28747](https://togithub.com/bitnami/charts/pull/28747))

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