Bump MSRV of core crates to 1.56.0 #2973

Job	Run time
cargo +1.56 build	1m 13s
cargo minimal-versions build	26s
cargo +stable build	30s
cargo +1.41.0 build (futures-{core, io, sink})	34s
cargo +beta build	39s
cargo test (ubuntu-latest)	2m 52s
cargo +nightly build	33s
cargo test (macos-latest)	6m 1s
cargo test (windows-latest)	4m 42s
cargo bench	52s
cargo test (ubuntu-latest, aarch64-unknown-linux-gnu)	3m 29s
cargo hack check --feature-powerset	2m 32s
cargo test (ubuntu-latest, armv7-unknown-linux-gnueabihf)	6m 4s
cargo miri test	14m 5s
cargo test (ubuntu-latest, armv5te-unknown-linux-gnueabi)	7m 39s
cargo build --target thumbv7m-none-eabi	25s
cargo test (ubuntu-latest, i686-unknown-linux-gnu)	3m 38s
cargo build --target thumbv6m-none-eabi	26s
cargo clippy	15s
cargo doc	18s
cargo fmt	2s
cargo test -Z sanitizer=address	3m 30s
cargo test -Z sanitizer=memory	5m 9s
cargo test -Z sanitizer=thread	2m 34s
	1h 8m 28s

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