Dependencies resolution with `--minimal-versions`

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Implementation PR: https://github.com/rust-lang/cargo/pull/5200
Docs: https://doc.rust-lang.org/nightly/cargo/reference/unstable.html#minimal-versions
Issues: https://github.com/rust-lang/cargo/labels/Z-minimal-versions

Steps:

• Document best-practices (https://github.com/rust-lang/cargo/issues/5656)

Unresolved questions:

• do we want to "impose" this feature on the ecosystem? Currently, everything seems to work fine due to eager dependency resolution. Adding --minimal-versions has costs: one-time ecosystem transition cost, cost to run CI job for minimal versions, cost to actually update minimal versions. There's anecdotal evidence that wrong minimal versions actually are a problem: https://www.reddit.com/r/rust/comments/8ob598/rust_minimum_versions_semver_is_a_lie/e027mtz/.

• should we implement "--minimal-versions-for-me-but-not-my-dependencies" as well, to make the initial roll-out of this feature easier?

Stabilization TODO:

• change -Z minimal-versions to just --minimal-versions and add it alongside --frozen/locked,
• this will require community-wide effort to make the things work, so a special announcement should be prepared. Announcement should describe the links problem and solution.

Start by reading the implementation PR #5200 and the unstable minimal-versions documentation, then review issue #5656 on best practices. The unresolved ecosystem and dependency-scope questions must be settled before stabilization; done means completing the listed stabilization TODO, including the flag change, placement beside --frozen/locked, and community announcement.