cleanup: Remove `rustversion(since(1.60))` tag #1225

Job	Run time
cargo check	14s
example - bytes	28s
example - diesel	23s
example - macros	16s
example - sqlx	21s
cargo miri test	4m 42s
cargo test -Zrandomize-layout	4m 22s
example - serde	16s
cargo test (stable)	2m 23s
example - traits	16s
cargo test (nightly)	1m 57s
	15m 38s

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