This is part of rust-lang#9930 for rust-lang/rfcs#3537

This will make it easier to maintain an MSRV-compliant `Cargo.toml` but
leaves validation up to the user as the resolver will pick newer
versions.
This helps the MSRV-aware workflows enumerated in
rust-lang/rfcs#3537 though it could be confusing to the workflow with an
MSRV-compatible lockfile.
PR rust-lang#13561 at least raises awareness of that discrepancy.

There is an unresolved question on differences in the resolver vs
`cargo add` for dealing with an unset `rust-version`.
However, we are only stabilizing the `cargo add` side which is a very
light two-way door as this is a UX-focused command and not a
programmatic command.

This hasn't gotten much end-user testing but, as its UX focused, that
seems fine.

As such, this seems like it is ready to stabilize.

This is part of rust-lang#9930 for rust-lang/rfcs#3537

This will make it easier to maintain an MSRV-compliant `Cargo.toml` but
leaves validation up to the user as the resolver will pick newer
versions.
This helps the MSRV-aware workflows enumerated in
rust-lang/rfcs#3537 though it could be confusing to the workflow with an
MSRV-compatible lockfile.
PR rust-lang#13561 at least raises awareness of that discrepancy.

There is an unresolved question on differences in the resolver vs
`cargo add` for dealing with an unset `rust-version`.
However, we are only stabilizing the `cargo add` side which is a very
light two-way door as this is a UX-focused command and not a
programmatic command.

This hasn't gotten much end-user testing but, as its UX focused, that
seems fine.

As such, this seems like it is ready to stabilize.

This is part of rust-lang#9930 for rust-lang/rfcs#3537

This will make it easier to maintain an MSRV-compliant `Cargo.toml` but
leaves validation up to the user as the resolver will pick newer
versions.
This helps the MSRV-aware workflows enumerated in
rust-lang/rfcs#3537 though it could be confusing to the workflow with an
MSRV-compatible lockfile.
PR rust-lang#13561 at least raises awareness of that discrepancy.

There is an unresolved question on differences in the resolver vs
`cargo add` for dealing with an unset `rust-version`.
However, we are only stabilizing the `cargo add` side which is a very
light two-way door as this is a UX-focused command and not a
programmatic command.

This hasn't gotten much end-user testing but, as its UX focused, that
seems fine.

As such, this seems like it is ready to stabilize.

feat(add): Stabilize MSRV-aware version req selection

### What does this PR try to resolve?

This is part of #9930 for rust-lang/rfcs#3537

This will make it easier to maintain an MSRV-compliant `Cargo.toml` but leaves validation up to the user as the resolver will pick newer versions.
This helps the MSRV-aware workflows enumerated in
rust-lang/rfcs#3537

### How should we test and review this PR?

As for determining if this is ready for stabilization:

By stabilizing this without the MSRV-aware resolver, this could be confusing to the workflow with an MSRV-compatible lockfile.
PR #13561 at least raises awareness of that discrepancy.
In general there was interest in the RFC discussions to stabilize this ASAP, regardless of what resolver direction we went.

There is an unresolved question on differences in the resolver vs `cargo add` for dealing with an unset `rust-version` (noted in the tracking issue). However, we are only stabilizing the `cargo add` side which is a very light two-way door as this is a UX-focused command and not a programmatic command.

This hasn't gotten much end-user acceptance testing but, as its UX focused, that seems fine (light, two way door)

As such, this seems like it is ready to stabilize.

### Additional information