This commit upgrades gRPC to protobuf v25.0 and makes some fixes to
account for upb changes. One major change is that upb has been merged
into the protobuf repo, so we can now drop the separate `@upb`
dependency. Another is that `.upb.c` files no longer exist and there are
new `.upb_minitable.h` and `.upb_minitable.c` files. The longer
filenames exceeded a Windows restriction, so to work around that I
renamed the `upb-generated` directory to just `upb-gen`, and likewise
for `upbdefs-generated`.
016ed921ce