fix(charts): update helm release grafana to 6.21.8

[renovate[bot]]

This PR contains the following updates:

Package	Update	Change
grafana (source)	patch	6.21.5 -> 6.21.8

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Release Notes

grafana/helm-charts

v6.21.8

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The leading tool for querying and visualizing time series and metrics.

v6.21.7

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The leading tool for querying and visualizing time series and metrics.

v6.21.6

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The leading tool for querying and visualizing time series and metrics.

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