Add dotnet9 to abi compat workflow (#13046)

.github/workflows/ci-compat.yml

@@ -16,6 +16,11 @@ jobs:
           ref: ${{ github.event.pull_request.head.sha }}
           repository: ${{ github.event.pull_request.head.repo.full_name }}
 
+      - name: Setup .NET
+        uses: actions/setup-dotnet@3e891b0cb619bf60e2c25674b222b8940e2c1c25 # v4.1.0
+        with:
+          dotnet-version: '9.0.x'
+
       - name: Build
         run: |
           dotnet build Jellyfin.Server -o ./out
@@ -41,6 +46,11 @@ jobs:
           repository: ${{ github.event.pull_request.head.repo.full_name }}
           fetch-depth: 0
 
+      - name: Setup .NET
+        uses: actions/setup-dotnet@3e891b0cb619bf60e2c25674b222b8940e2c1c25 # v4.1.0
+        with:
+          dotnet-version: '9.0.x'
+
       - name: Checkout common ancestor
         env:
           HEAD_REF: ${{ github.head_ref }}