chore(deps): update actions/checkout action to v6 (#168)

This PR contains the following updates:

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

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### Release Notes

<details>
<summary>actions/checkout (actions/checkout)</summary>

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

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

</details>

---

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are satisfied.

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---

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Author: scotwells

.github/workflows/testing.yaml

@@ -7,7 +7,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Install dependencies
         run: go mod download
       - name: Execute golang tests