Enforce Rust edition 2021 (#3332)
* Rust edition 2021: zebra-network, cargo fix --edition and clippy --fix
* Rust edition 2021: zebra-chain, cargo fix --edition
* Rust edition 2021: tower-batch, cargo fix --edition
* Rust edition 2021: tower-fallback, cargo fix --edition
* Rust edition 2021: zebra-client, cargo fix --edition
* Rust edition 2021: zebra-consensus, cargo fix --edition
* Rust edition 2021: zebra-rpc, cargo fix --edition
* Rust edition 2021: zebra-state, cargo fix --edition
* Rust edition 2021: zebra-state, cargo fix --edition
* Rust edition 2021: zebra-test, cargo fix --edition
* Rust edition 2021: zebra-utils, cargo fix --edition
* Rust edition 2021: zebrad, cargo fix --edition
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
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