Update playground automatically upon new ink! release

[HCastano]

I think it would be cool to have some sort of service that subscribes via webhooks to new
ink! and/or cargo-contract releases and automatically updates the compiler and
contract TOML files for the new release. It can do something like grab the latest
template generated by cargo contract new and use that.

A prerequisite for this is a CI job which checks if the playground can still build
contracts when these dependencies get updated.