Add global.json for .NET SDK version management and remove hardcoded version in setup

Author: Søren Trudsø Mahon

.github/workflows/build.dotnet.testcontainers.yml

@@ -16,8 +16,6 @@ jobs:
 
             - name: Setup .NET
               uses: actions/setup-dotnet@v4
-              with:
-                  dotnet-version: "8.0.x"
 
             - name: Install .NET Aspire workload
               run: dotnet workload install aspire

.github/workflows/build.dotnet.yml

@@ -19,8 +19,6 @@ jobs:
 
             - name: Setup .NET
               uses: actions/setup-dotnet@v4
-              with:
-                  dotnet-version: "8.0.x"
 
             - name: Install .NET Aspire workload
               run: dotnet workload install aspire

global.json

@@ -0,0 +1,6 @@
+{
+  "sdk": {
+      "version": "8.0.0",
+      "rollForward": "minor"
+  }
+}