Graphite dependency no longer hosted by kiwigrid

As seen [here](https://github.com/Flagsmith/flagsmith-charts/actions/runs/19325686789) (full traceback also pasted below), kiwigrid have archived their helm charts repo [here](https://github.com/kiwigrid/helm-charts). This means that kiwigrid.github.io is no longer available, and hence our CI is failing.

```
Linting charts...

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Charts to be processed:
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flagsmith => (version: "0.77.3", path: "charts/flagsmith")
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"stable" has been added to your repositories
"bitnami" has been added to your repositories
"influxdata" has been added to your repositories
Error: failed linting charts: failed adding repo: kiwigrid=[https://kiwigrid.github.io:](https://kiwigrid.github.io/) failed waiting for process: exit status 1
Error: looks like "https://kiwigrid.github.io/" is not a valid chart repository or cannot be reached: failed to fetch https://kiwigrid.github.io/index.yaml : 404 Not Found

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No chart changes detected.
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failed linting charts: failed adding repo: kiwigrid=[https://kiwigrid.github.io:](https://kiwigrid.github.io/) failed waiting for process: exit status 1
Error: Process completed with exit code 1.
```