Update ghcr.io/basecamp/kamal Docker tag to v2 (#295)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| ghcr.io/basecamp/kamal | final | major | `v1.9.2` -> `v2.4.0` |

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kamal/Dockerfile

@@ -1,4 +1,4 @@
-FROM ghcr.io/basecamp/kamal:v1.9.2
+FROM ghcr.io/basecamp/kamal:v2.4.0
 
 RUN apk --no-cache add shadow