Update actions/checkout action to v6 (#24)

| datasource  | package          | from | to |
| ----------- | ---------------- | ---- | -- |
| github-tags | actions/checkout | v5   | v6 |

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

Author: renovate[bot]

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
           - "8.4"
 
     steps:
-      - uses: "actions/checkout@v5"
+      - uses: "actions/checkout@v6"
       - uses: "shivammathur/setup-php@v2"
         with:
           coverage: "none"
@@ -48,7 +48,7 @@ jobs:
             code-coverage: "pcov"
 
     steps:
-      - uses: "actions/checkout@v5"
+      - uses: "actions/checkout@v6"
       - uses: "shivammathur/setup-php@v2"
         with:
           coverage: "${{ matrix.code-coverage }}"
@@ -81,7 +81,7 @@ jobs:
           - "8.4"
 
     steps:
-      - uses: "actions/checkout@v5"
+      - uses: "actions/checkout@v6"
       - uses: "shivammathur/setup-php@v2"
         with:
           coverage: "none"