Latest ahash dep and Rust 1.70.0 are incompatible

[frewsxcv]

error: package `ahash v0.8.8` cannot be built because it requires rustc 1.72.0 or newer, while the currently active rustc version is 1.70.0

See #1105

[urschrei]

Do we commit to three or four stable versions, or a duration, or both? 1.72 is six months old on the 24th of Feb.

[lnicola]

4 versions, or around 6 months. That's a MSRV of 1.73.

[frewsxcv]

Although we can bump our MSRV, I think it'd be easier to pin ahash to a specific version until we have a real need to bump the MSRV for a new feature

[frewsxcv]

This was resolved upstream tkaitchuck/aHash#208