Update .bazelignore to ignore the upb/ directory

upb is going to have its own Bazel repository for now, so this change will
ensure that things like `bazel query //...` won't try to descend into upb and
get confused. I would think that Bazel should see the `WORKSPACE` file and
know not to descend into the upb/ directory, but that does not seem to be
happening (possibly a Bazel bug).

PiperOrigin-RevId: 560072475

.bazelignore

@@ -1,4 +1,6 @@
 # These are fetched as external repositories.
 third_party/abseil-cpp
 third_party/googletest
+# upb is its own Bazel repository for now.
+upb/
 _build/