Update CI

Author: FlorianRappl

.github/workflows/ci.yml

@@ -53,9 +53,9 @@ jobs:
     - uses: actions/setup-dotnet@v1
       with:
         dotnet-version: | 
-          3.1.x
-          5.0.x
           6.0.x
+          7.0.x
+          8.0.x
 
     - name: Build
       run: ./build.sh
@@ -69,9 +69,9 @@ jobs:
     - uses: actions/setup-dotnet@v1
       with:
         dotnet-version: | 
-          3.1.x
-          5.0.x
           6.0.x
+          7.0.x
+          8.0.x
 
     - name: Build
       run: |