Bump hashtables upper version bounds

Allow `agda` to build with the latest version of `hashtables` (1.2.0.0). As far as I can tell, [this release](https://github.com/gregorycollins/hashtables/blob/d695f322a6465711ab7b43d4b41f5365255f0912/changelog.md#1200) did not introduce any API changes that would affect `agda`.