Upgrade shellcheck checkout to make it work again. (DOMjudge#96)

See
https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/
and
actions/checkout#1047

Author: meisterT

.github/workflows/shellcheck.yml

@@ -7,7 +7,7 @@ jobs:
     name: Shellcheck
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - name: Run ShellCheck
         uses: ludeeus/action-shellcheck@master
         env: