.github/workflows: Bump actions/checkout from v3 to v4

`checkout@v3` makes use of Node 16, which has reached end of life [1].
`checkout@v4` makes use of Node 20 instead [2,3].

[1] https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/
[2] https://github.com/actions/checkout/blob/main/CHANGELOG.md#v400
[3] actions/checkout#1436

#1459

Signed-off-by: Dominique Polek <dominique.polek@mailbox.org>

.github/workflows/ubuntu-tests.yaml

@@ -29,7 +29,7 @@ jobs:
 
     steps:
       - name: Checkout
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           path: containers/toolbox
           submodules: true
@@ -59,7 +59,7 @@ jobs:
           echo "PATH=/usr/lib/go-1.20/bin:$PATH" >> "$GITHUB_ENV"
 
       - name: Checkout Bats
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           path: bats-core/bats-core
           ref: v1.10.0
@@ -78,7 +78,7 @@ jobs:
         working-directory: bats-core/bats-core
 
       - name: Checkout shadow
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
         with:
           path: shadow-maint/shadow
           ref: 4.13