Update web-sys dependency to not allow v0.4 and above

[jplatte]

This was added in #621 (comment), but I think the author misunderstood what this constraint means.

In Cargo, a version specification like 0.5.1 means >=0.5.1 <0.6.0 (like ^0.5.1 in NPM, it can also be written that way in Cargo if you want to be more explicit). A version specification like >=0.5.1 puts no upper bound, so even a breaking-change release like 0.7.x or 0.8.x would get selected.

This is documented in detail here: https://doc.rust-lang.org/cargo/reference/specifying-dependencies.html#specifying-dependencies-from-cratesio

cc @kflansburg