Migrate all rust-lang code to edition 2021

[est31]

8 months after the 2021 edition release, there are still many mentions of the 2018 edition on the official rust-lang organization according to this search.
I wonder if it might be a good idea to migrate them to the new edition or to keep them as is. Some probably can't be migrated due to MSRV concerns, but many others it is probably okay to migrate them and provide the best contribution experience.

cc #88637 and #88638 and which tracked this for the compiler and libraries in this repo.

cc @m-ou-se

@rustbot label A-edition-2021