Upgrade to Yew 0.21

[isosphere]

Tested locally, no trouble after running cargo update.

If you don't run cargo update, you might encounter this issue: https://github.com/yewstack/yew/issues/3366 - I did.

Given the upstream version change + breaking changes in #30, I'd like to combine them both in our next release.

This might be a good time to revisit #23.

[isosphere]

https://yew.rs/docs/migration-guides/yew/from-0_20_0-to-0_21_0

[isosphere]

I have reviewed the migration list. There's no problem for us, we don't implement the affected hooks at this time.