Update MSRV to 1.56

[teoxoy]

As mentioned on matrix, wgpu's MSRV has been recently bumped up to 1.59 and I proposed to bump the MSRV of naga to 1.56.

Besides API changes, I think these are the most relevant features for us since 1.43 (our current MSRV):

• 1.56; rust 2021 (can upgrade in the future), rust-version in Cargo.toml, multiple bindings in match arms
• 1.55; faster, more correct float parsing
• 1.53; ability to nest OR patterns in match arms
• 1.51; Const Generics MVP
• 1.50; const-generic array indexing, const value repetition for arrays
• 1.46; const fn improvements

Tested changes locally via cargo msrv verify -- cargo test --all-features --workspace (cargo-msrv).