Add Bazel 9.x to our CI matrix

Use the latest minor/patch release so we notice breakages (like the
recent CcInfo/golang issue) immediately.

Author: jdcormie

.github/workflows/testing.yml

@@ -80,8 +80,12 @@ jobs:
     strategy:
       matrix:
         bzlmod: [true, false]
+        bazel_version: [8.x, 9.x]
+        exclude:
+          - bazel_version: 9.x
+            bzlmod: false
     env:
-      USE_BAZEL_VERSION: 8.7.0
+      USE_BAZEL_VERSION: ${{ matrix.bazel_version }}
 
     steps:
     - uses: actions/checkout@v4