Update module code.gitea.io/sdk/gitea to v0.21.0 (forgejo) (#7458)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| code.gitea.io/sdk/gitea | require | minor | `v0.20.0` -> `v0.21.0` |

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Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7458
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>