Update GitHub Actions to more recent versions

This should fix the error "Could not find '.NET Core SDK' with version
= 2.1.818" thrown by `dotnet_install` during GitHub Actions (CI).

Author: stakx

.github/workflows/build.yml

@@ -19,31 +19,31 @@ jobs:
     steps:
 
       - name: Checkout repository
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
       # Several .NET Core versions will be used during the test run.
       # The lowest version gets installed first in order to prevent
       # "a newer version is already installed" install errors.
 
       - name: Install .NET Core 2.1
-        uses: actions/setup-dotnet@v1
+        uses: actions/setup-dotnet@v4
         with:
           dotnet-version: 2.1.x
 
       - name: Install .NET Core 3.1
-        uses: actions/setup-dotnet@v1
+        uses: actions/setup-dotnet@v4
         with:
           dotnet-version: 3.1.x
 
       - name: Install .NET 6.0
-        uses: actions/setup-dotnet@v1
+        uses: actions/setup-dotnet@v4
         with:
           dotnet-version: 6.0.x
 
       # Building requires an up-to-date .NET SDK.
 
       - name: Install .NET 7.0
-        uses: actions/setup-dotnet@v1
+        uses: actions/setup-dotnet@v4
         with:
           dotnet-version: 7.0.x