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`.
This is to help simplify the grpc iOS release flow.
Package.swift was manually written and out of sync for a long time in
this repo.
They are currently being updated updated for releases in
[grpc-ios](https://github.com/grpc/grpc-ios) repo, which includes a lot
of manual work to include new files and exclude tests.
This diff generate Package.swift from a template like other languages to
avoid the tedious work. Currently only gRPC-Core is generated form the
template, gRPC-cpp remains the same for now as they don't change often.
CC @sampajano
Hannah Shi
Adam Cozzette