Update actions/checkout action to v5 (#8)

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

Author: renovate[bot]

.github/workflows/ci.yml

@@ -11,7 +11,7 @@ jobs:
         os:   [ 'ubuntu', 'macos', 'windows' ]
         ruby: [ '3.2', '3.3', '3.4' ]
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}

.github/workflows/doc.yml

@@ -10,7 +10,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v5
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ruby