Move .NET version override in CI

Author: cretz

.github/workflows/ci.yml

@@ -22,6 +22,9 @@ jobs:
           - os: ubuntu-latest
             docsTarget: true
             cloudTestTarget: true
+            # This is here alongside docsTarget because newer docfx doesn't work
+            # with .NET 6.
+            dotNetVersionOverride: 8.x
           - os: ubuntu-arm
             runsOn: ubuntu-24.04-arm64-2-core
           - os: macos-intel