Bump CI to use 3.9.0

Author: srawlins

.github/workflows/test.yaml

@@ -22,7 +22,7 @@ jobs:
       fail-fast: false
       matrix:
         os: [ubuntu-latest]
-        sdk: [3.8.0, dev]
+        sdk: [3.9.0, dev]
         job: [main, flutter, packages, sdk-docs]
         include:
           - os: macos-latest