Update MSRV to 1.69.0 and update flake #71

Job	Run time
(Windows) Builds (x86_64-pc-windows-msvc)	58s
Bench Compile (stable)	1m 51s
(OS X) Builds (aarch64-apple-ios)	3m 4s
(Linux) Builds (aarch64-linux-android)	2m 8s
(OS X) Builds (x86_64-apple-darwin)	2m 12s
Check (stable)	21s
(Linux) Builds (x86_64-unknown-linux-musl)	2m 0s
Check (beta)	17s
(Linux) Builds (wasm32-unknown-unknown)	1m 56s
Rustfmt (nightly)	16s
Check (1.69.0)	3m 30s
Test Suite (stable)	1m 49s
Test Suite (1.60.0)	3m 24s
	23m 46s

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