Rollup merge of #131289 - RalfJung:duration_consts_float, r=tgross35

stabilize duration_consts_float

Waiting for FCP in rust-lang/rust#72440 to pass.

`as_millis_f32` and `as_millis_f64` are not stable at all yet, so I moved their const-stability together with their regular stability (tracked at rust-lang/rust#122451).

Fixes rust-lang/rust#72440