Although I don't know why, mirage-crypo-rng package of >= 2.0.0 is released
but mirage-crypto-rng-eio is not (it's stuck at 1.2.0).
cf. https://opam.ocaml.org/packages/mirage-crypto-rng/mirage-crypto-rng.2.0.3/
cf. https://opam.ocaml.org/packages/mirage-crypto-rng-eio/mirage-crypto-rng-eio.1.2.0/
Currently, ocaml-opentelemetry depends on mirage-crypto-rng-eio, so
those who want to use mirage-crypto-rng >= 2.0.0 (like me) can't use
ocaml-opentelemetry.
This commit fixes this problem by replacing mirage-crypto-rng-eio with
mirage-crypto-rng in dune-project (thus in
opentelemetry-client-cohttp-eio.opam) to make ocaml-opentelemetry depend
on mirage-crypto-rng instead of mirage-crypto-rng-eio.
