ci: bump actions off the deprecated Node 20 runtime (#4)

checkout v4 -> v6, setup-go v5 -> v6, upload-artifact v4 -> v7 (current latest
majors, Node 24). Clears the Node 20 deprecation warning. Workflow structure
unchanged.

Author: Sbcdn

.github/workflows/ci.yml

@@ -16,10 +16,10 @@ jobs:
     timeout-minutes: 15
     steps:
       - name: Check out
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@v6
         with:
           go-version: '1.25.7'
           cache: true
@@ -60,10 +60,10 @@ jobs:
     timeout-minutes: 30
     steps:
       - name: Check out
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
 
       - name: Set up Go
-        uses: actions/setup-go@v5
+        uses: actions/setup-go@v6
         with:
           go-version: '1.25.7'
           cache: true
@@ -73,7 +73,7 @@ jobs:
 
       - name: Upload Cardano artifacts on failure
         if: failure()
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v7
         with:
           name: out-on-failure
           path: out/