chore: Update actions/checkout action to v6 (#87)

This PR contains the following updates:

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

---

### 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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---

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.github/workflows/checks.yml

@@ -14,7 +14,7 @@ jobs:
       GO111MODULE: on
     steps:
       - name: Check out code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - uses: actions/setup-go@v6
         with:
           go-version-file: go.mod

.github/workflows/release-drafter.yml

@@ -25,7 +25,7 @@ jobs:
       GITHUB_TOKEN: ${{ secrets.RELEASE_LABELER_TOKEN }}
     steps:
       - name: Checkout code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           ref: main
           fetch-depth: 0

.github/workflows/release.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout Source
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
         with:
           fetch-depth: 0
       - name: Set up Go

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Install Neovim
         uses: rhysd/action-setup-vim@v1
         with:

.github/workflows/vulns.yml

@@ -15,7 +15,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Check out code
-        uses: actions/checkout@v5
+        uses: actions/checkout@v6
       - name: Setup Golang
         uses: actions/setup-go@v6
         with: