fix: add bazel versions tested to presubmit (#202)

See:
bazelbuild/bazel-central-registry#1332

Author: thesayyn

.bcr/presubmit.yml

@@ -1,8 +1,10 @@
 matrix:
   platform: ["centos7", "debian10", "macos", "ubuntu2004", "windows"]
+  bazel: ['7.x', '6.x']
 tasks:
   verify_build_targets:
     name: "Verify build targets"
+    bazel: ${{ bazel }}
     platform: ${{ platform }}
     build_targets:
       - "@rules_proto//proto/..."