CI: Update checkout actions

Author: bquorning

.github/workflows/ci.yml

@@ -18,7 +18,7 @@ concurrency:
 jobs:
   rubocop:
     name: Rubocop
-    runs-on: 'ubuntu-20.04'
+    runs-on: ubuntu-20.04
     steps:
       - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
@@ -188,7 +188,7 @@ jobs:
 
     env: ${{ matrix.env }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - uses: ruby/setup-ruby@v1
         with:
           ruby-version: ${{ matrix.ruby }}
@@ -230,7 +230,7 @@ jobs:
       JRUBY_OPTS: ${{ matrix.container.jruby_opts || '--dev' }}
       RAILS_VERSION: ${{ matrix.env.RAILS_VERSION }}
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
       - run: apt-get install libsqlite3-dev
       - run: ${{ matrix.container.pre }}
       - run: script/legacy_setup.sh