chore(deps): Update actions/setup-go action to v6 (#468)

> [!NOTE]
> Mend has cancelled [the proposed
renaming](https://redirect.github.com/renovatebot/renovate/discussions/37842)
of the Renovate GitHub app being renamed to `mend[bot]`.
> 
> This notice will be removed on 2025-10-07.

<hr>

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/setup-go](https://redirect.github.com/actions/setup-go) |
action | major | `v5` -> `v6` |

---

### Release Notes

<details>
<summary>actions/setup-go (actions/setup-go)</summary>

### [`v6`](https://redirect.github.com/actions/setup-go/compare/v5...v6)

[Compare
Source](https://redirect.github.com/actions/setup-go/compare/v5...v6)

</details>

---

### Configuration

📅 **Schedule**: Branch creation - Between 12:00 AM and 03:59 AM, on day
1 of the month ( * 0-3 1 * * ) (UTC), Automerge - At any time (no
schedule defined).

🚦 **Automerge**: Enabled.

♻ **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.

---

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

---

This PR was generated by [Mend Renovate](https://mend.io/renovate/).
View the [repository job
log](https://developer.mend.io/github/hypermodeinc/ristretto).

<!--renovate-debug:eyJjcmVhdGVkSW5WZXIiOiI0MS4xMzEuOSIsInVwZGF0ZWRJblZlciI6IjQxLjEzMS45IiwidGFyZ2V0QnJhbmNoIjoibWFpbiIsImxhYmVscyI6W119-->

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci-ristretto-tests.yml

@@ -23,7 +23,7 @@ jobs:
     steps:
       - uses: actions/checkout@v5
       - name: Setup Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@v6
         with:
           go-version-file: go.mod
       - name: Run Unit Tests