Bump MSRV of core crates to 1.56.0 #2981

Job	Run time
cargo +1.56.0 build (futures-{core, io, sink})	34s
cargo bench	45s
cargo +stable build	32s
cargo minimal-versions build	26s
cargo hack check --feature-powerset	2m 20s
cargo +beta build	39s
cargo miri test	14m 19s
cargo +nightly build	34s
cargo build --target thumbv7m-none-eabi	27s
cargo test -Z sanitizer=address	3m 26s
cargo build --target thumbv6m-none-eabi	27s
cargo test (ubuntu-latest)	2m 37s
cargo test -Z sanitizer=memory	5m 1s
cargo +1.56 build	1m 11s
cargo test (macos-latest)	6m 14s
cargo test -Z sanitizer=thread	2m 36s
cargo test (windows-latest)	4m 43s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 39s
cargo clippy	15s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 7s
cargo doc	17s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	6m 27s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 17s
cargo fmt	3s
	1h 6m 56s

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