chore(BCR): drop Bazel 6 testing (#397)

it's red for the last couple releases, and Bazel 8 RC will come out next
week so we should switch to testing that.

Author: alexeagle

.bcr/presubmit.yml

@@ -1,7 +1,7 @@
 bcr_test_module:
   module_path: "e2e/use_release"
   matrix:
-    bazel: ["7.x", "6.x"]
+    bazel: ["7.x"]
     # TODO(#9): add windows to this list
     platform: ["debian10", "macos", "ubuntu2004"]
   tasks: