Update checkout action to version 5 (#9)

Author: tassoman

.github/workflows/publiccode-validation.yml

@@ -5,7 +5,7 @@ jobs:
     runs-on: ubuntu-latest
     name: publiccode validation
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v5
     - uses: italia/publiccode-parser-action@v1
       with:
         publiccode: 'publiccode.yml' # relative path to your publiccode.yml