tests(bcr): Set Bazel version in BCR presubmit config. (bazelbuild#1753)

BCR now requires the presubmit configs to specify the Bazel versions to
run tests with. Without this, the BCR checks fail and we can't do
releases to BCR.

.bcr/presubmit.yml

@@ -16,9 +16,11 @@ bcr_test_module:
  module_path: "examples/bzlmod"
  matrix:
  platform: ["debian11", "macos", "ubuntu2004", "windows"]
+ bazel: [6.x, 7.x]
  tasks:
  run_tests:
  name: "Run test module"
  platform: ${{ platform }}
+ bazel: ${{ bazel }}
  test_targets:
  - "//..."