Update alpine Docker tag to v3.21.0 (petejohanson#6)

This PR contains the following updates:

| Package | Update | Change |
|---|---|---|
| alpine | minor | `3.20.3` -> `3.21.0` |

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### Configuration

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🚦 **Automerge**: Disabled by config. Please merge this manually once you are satisfied.

♻ **Rebasing**: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

🔕 **Ignore**: Close this PR and you won't be reminded about this update again.

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 - [ ] <!-- rebase-check -->If you want to rebase/retry this PR, check this box

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Reviewed-on: https://git.chdorner.com/chdorner/panic34/pulls/6
Co-authored-by: Renovate Bot <[email protected]>
Co-committed-by: Renovate Bot <[email protected]>

.woodpecker/build.yml

@@ -20,7 +20,7 @@ steps:
       # - west build -s zmk/app -b seeeduino_xiao_ble -d build/zaphod_lite_xiao_ble -- -DZMK_CONFIG="$CI_WORKSPACE/config" -DSHIELD=zaphod_lite
       - cat -n build/zaphod/zephyr/.config
   - name: upload
-    image: alpine:3.20.3@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a
+    image: alpine:3.21.0@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45
     commands:
       - apk add curl
       - stat build/zaphod/zephyr/zmk.uf2