feat(charts): update helm chart thanos to 12.20.0

| datasource | package | from    | to      |
| ---------- | ------- | ------- | ------- |
| helm       | thanos  | 12.18.0 | 12.20.0 |


Signed-off-by: Jeff Billimek <billimek@users.noreply.github.com>

monitoring/thanos/thanos.yaml

@@ -10,7 +10,7 @@ spec:
     spec:
       # renovate: registryUrl=https://charts.bitnami.com/bitnami
       chart: thanos
-      version: 12.18.0
+      version: 12.20.0
       sourceRef:
         kind: HelmRepository
         name: bitnami-charts