Auto merge of #4920 - wking:license-expression, r=matklad

Auto merge of #4920 - wking:license-expression, r=matklad

Cargo.toml: Replace '/' with 'OR' in 'license'

Catch up with our recommendations from 7dee65fe (#4898), which deprecated `/` in favor of vanilla SPDX license expressions.

I've gone with the disjunctive `OR`, because the README has:

> Cargo is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0).