feat(charts): update helm release thanos to v15.6.0

[renovate[bot]]

This PR contains the following updates:

Package	Update	Change
thanos (source)	minor	15.5.1 -> 15.6.0

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Release Notes

bitnami/charts (thanos)

### [`v15.6.0`](https://togithub.com/bitnami/charts/blob/HEAD/bitnami/thanos/CHANGELOG.md#1560-2024-05-30) - \[bitnami/thanos] Receive, ruler & storegateway statefulsets persistentVolumeClaimRetentionPolicy support ([#​25676](https://togithub.com/bitnami/charts/pull/25676))

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