chore: bump MSRV to 1.90

[DaniPopes]

Replace the manual const reverse loop in to_be_bytes with bytes.reverse(), which is const-stable as of Rust 1.90.

[prestwich]

should we prep a minor bump?

[codspeed-hq]

Merging this PR will improve performance by 11%

⚡ 3 improved benchmarks
✅ 383 untouched benchmarks

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	most_significant_bits/4096/4096	28.9 µs	26.1 µs	+11%
⚡	to/f32/4096	29.1 µs	26.3 µs	+10.77%
⚡	to/f64/4096	29.2 µs	26.3 µs	+10.85%

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_{Comparing DaniPopes:dani/msrv-1.90 (313b839) with main (403227f)}