Update actions/checkout action to v6 (#1849)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [actions/checkout](https://redirect.github.com/actions/checkout) |
action | major | [`v4` ->
`v6`](https://redirect.github.com/actions/checkout/compare/v4...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)

### [`v5`](https://redirect.github.com/actions/checkout/compare/v4...v5)

[Compare
Source](https://redirect.github.com/actions/checkout/compare/v4...v5)

</details>

---

### Configuration

📅 **Schedule**: Branch creation - "after 8pm,before 6am" in timezone
America/Los_Angeles, Automerge - "after 8pm,before 6am" in timezone
America/Los_Angeles.

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

♻ **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

---

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.github/workflows/hakari.yml

@@ -15,7 +15,7 @@ jobs:
     env:
       RUSTFLAGS: -D warnings
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
       - name: Install cargo-hakari
         uses: taiki-e/install-action@v2
         with:

.github/workflows/rust.yml

@@ -13,21 +13,21 @@ jobs:
   check-style:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+    - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6
     - name: Report rustfmt version
       run: cargo fmt -- --version
     - name: Check style
       run: cargo fmt -- --check
   build-docs:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+    - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6
     - name: Test build documentation
       run: cargo doc
   build:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+    - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6
       with:
         # Fetch all history: the Dropshot API up-to-date check requires it.
         fetch-depth: 0
@@ -38,6 +38,6 @@ jobs:
   clippy:
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@34e114876b0b11c390a56381ad16ebd13914f8d5 # v4
+    - uses: actions/checkout@1af3b93b6815bc44a9784bd300feb67ff0d1eeb3 # v6
     - name: Test Libraries
       run: cargo clippy --all-targets