Auto merge of #3497 - jtgeibel:heroku-20, r=Turbo87
Auto merge of #3497 - jtgeibel:heroku-20, r=Turbo87

Bump to ubuntu-20.04 on CI

We can merge this after bumping to the heroku-20 stack on production.

r? `@ghost`

rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io,rust-lang/crates.io