chore(deps): update actions/attest-build-provenance action to v4 (#62)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/attest-build-provenance](https://github.com/actions/attest-build-provenance) | action | major | `v3` → `v4` |

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### Release Notes

<details>
<summary>actions/attest-build-provenance (actions/attest-build-provenance)</summary>

### [`v4`](https://github.com/actions/attest-build-provenance/compare/v3...v4)

[Compare Source](https://github.com/actions/attest-build-provenance/compare/v3...v4)

</details>

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### Configuration

📅 **Schedule**: Branch creation - At any time (no schedule defined), Automerge - At any time (no schedule defined).

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

♻ **Rebasing**: Whenever PR is behind base branch, or you tick the rebase/retry checkbox.

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

---

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

---

This PR has been generated by [Renovate Bot](https://github.com/renovatebot/renovate).
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Reviewed-on: #62
Co-authored-by: renovate[bot] <renovate-bot@git.tainton.uk>
Co-committed-by: renovate[bot] <renovate-bot@git.tainton.uk>

.github/workflows/build-push-attest-docker.yml

@@ -59,7 +59,7 @@ jobs:
             ghcr.io/${{ github.repository }}:${{ inputs.release }}
 
       - name: Generate artifact attestation
-        uses: actions/attest-build-provenance@v3
+        uses: actions/attest-build-provenance@v4
         with:
           subject-name: ghcr.io/${{ github.repository }}
           subject-digest: ${{ steps.push.outputs.digest }}