Update actions/checkout action to v4

Author: renovate[bot]

.github/workflows/continuous-integration.yml

@@ -14,7 +14,7 @@ jobs:
         node: [ '12', '13', '14' ]
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - uses: actions/cache@v2
         with:
@@ -44,7 +44,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
 
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
 
       - uses: actions/cache@v2
         with: