Update data.forgejo.org/oci/alpine Docker tag to v3.21 (forgejo) (#6502)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [data.forgejo.org/oci/alpine](https://hub.docker.com/_/alpine) ([source](https://github.com/alpinelinux/docker-alpine)) | final | minor | `3.20` -> `3.21` |

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Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6502
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Renovate Bot <forgejo-renovate-action@forgejo.org>
Co-committed-by: Renovate Bot <forgejo-renovate-action@forgejo.org>

.forgejo/testdata/build-release/Dockerfile

@@ -1,4 +1,4 @@
-FROM data.forgejo.org/oci/alpine:3.20
+FROM data.forgejo.org/oci/alpine:3.21
 ARG RELEASE_VERSION=unkown
 LABEL maintainer="contact@forgejo.org" \
       org.opencontainers.image.version="${RELEASE_VERSION}"