Allow setting `edition = "2018"` in a workspace

[Centril]

For larger projects, it could be useful to set the edition once instead of in each project's Cargo.toml file. This would then be equivalent to setting the edition key in every member project in the workspace.

Currently, the edition key is ignored instead: #5726

See also: rust-lang/edition-guide#45

[dwijnand]

Like #5661, but less granular 🙂.

(I mention it also because one might want to tackle these in one swing.)

[lucksus]

Especially since cargo fix --edition updates every crate in the whole workspace.

[pksunkara]

With the new edition coming next year, we should look into implementing this. @ehuss Are there any blockers for this?

[ehuss]

I believe it is being implemented as part of #8664.

[futile]

#9684 seems to be the relevant PR now. Also bump, since the 2021 edition is coming up, and this could be relevant for migration.