Bump GitHub Actions Checkout

This avoids deprecation of Node 16 which is likely to come soon
given that it is now beyond end of life.

Author: PeterJCLaw

.github/workflows/main.yml

@@ -27,7 +27,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout sources
-        uses: actions/checkout@v3
+        uses: actions/checkout@v4
 
       - uses: actions/setup-node@v2
         with:
@@ -43,7 +43,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v3
+      - uses: actions/checkout@v4
         with:
           submodules: recursive