Fix dependencies for eio and async

[quernd]

We don't require the h2-{async,eio,lwt} packages anymore in the .opam files, so we can't have them in the dune files either.

This was inadvertently broken when making the dependencies more consistent. It's hard to find in OCaml-CI because it installs the dependencies of all packages first (and the examples pull in everything):

RUN opam update --depexts && opam install --cli=2.1 --depext-only -y grpc.dev grpc-lwt.dev grpc-examples.dev grpc-eio.dev grpc-bench.dev grpc-async.dev $DEPS
RUN opam install $DEPS
COPY --chown=1000:1000 . /src
RUN opam exec -- dune build @install @check @runtest && rm -rf _build

(But it caused the Windows workflow to fail.)

OPAM in contrast checks package by package. Can we do that too?

[tmcgilchrist]

@quernd I added additional fixes for the example code and lower bounds on eio. Once it passes CI I will merge it.
Thanks.

[quernd]

Thanks @tmcgilchrist! I don't know how I missed that, I could swear it was all green 🤔