Update CI workflow for .NET 9

Author: Kaliumhexacyanoferrat

.github/workflows/ci.yml

@@ -1,6 +1,9 @@
 name: CI
 
-on: [push, pull_request]
+on: 
+  push:
+  pull_request:
+    types: [opened]
 
 jobs:
   build:
@@ -18,14 +21,9 @@ jobs:
       with:
         fetch-depth: 0
     - name: Setup .NET
-      uses: actions/setup-dotnet@v1
+      uses: actions/setup-dotnet@v4.1.0
       with:
-        dotnet-version: 8.0.100
-    - name: Setup Java JDK
-      uses: actions/setup-java@v3
-      with:
-        distribution: temurin
-        java-version: 21
+        dotnet-version: 9.0
     - name: Restore tools
       run: dotnet tool restore
     - name: Begin scan