`{f32,f64}::round_ties_even` is stable as of Rust 1.77

[ErichDonGubler]

Const. eval. code currently uses a hand-rolled version of {f32,f64}::round_ties_even. Once our MSRV becomes Rust 1.77 or later, we should use the stabilized implementation in std instead.

• Rust upstream PR: Stabilize round_ties_even rust-lang/rust#120150
• https://doc.rust-lang.org/stable/std/primitive.f64.html#method.round_ties_even
• https://doc.rust-lang.org/stable/std/primitive.f32.html#method.round_ties_even