Adjust OR_PATTERNS_BACK_COMPAT lint level #84869
Description
The OR_PATTERNS_BACK_COMPAT lint level should be adjusted for the 2021 edition. Either the lint should be warn-by-default always, or it should be a 2021 future-incompatible lint. This is needed to smoothly migrate projects to 2021.
There is some appeal to making it always warn for less confusion, as long as the lint works well. I think this course needs to be overridable via 2021 migration (see https://github.com/orgs/rust-lang/projects/7#card-59862192), and be aware that projects which support a wide range of rustc versions may be forced to allow it.