chore: remove Bazel 6 testing

Our e2e only works when the new incompatible_enable_proto_toolchain_resolution flag is available

Author: alexeagle

.bcr/presubmit.yml

@@ -2,7 +2,7 @@ bcr_test_module:
   module_path: "e2e/smoke"
   matrix:
     platform: ["debian10", "macos", "ubuntu2004", "windows"]
-    bazel: ["7.x", "6.x"]
+    bazel: ["7.x"]
   tasks:
     run_tests:
       name: "Run test module"