Relax MSRV policy to exclude dependencies

[Emoun]

Short Description:

Relax MSRV policy to exclude any guarantees about the MSRV of duplicate's dependencies.

Motivation:

#42 showed that the current MSRV policy implementation of pinning any dependencies that don't have a compatible MSRV policy puts undue limits on users who want to use the same dependencies as duplicate.
The discussion in the same issue shows the problem isn't likely to be fixed by rust tooling, meaning pinning versions must be avoided by duplicate

As such, the MSRV is practically too restrictive and should be relaxed.

Design

The original vision of the MSRV was to promise that increasing it would always follow with a major version bump.
While this can be maintained by duplicate itself, it is unlikely to be a policy that is generally used by dependencies.

Therefore, to maintain as much of the promise as we can, while avoiding having to pin dependencies, the new policy is that dependencies don't necessarily have a compatible MSRV policy.
Instead, we will promise that the lowest version used by duplicate works for the MSRV. The user must then independently pin any dependencies they need that releases a version with a higher MSRV.

Misc:

[Emoun]

To practically test the new policy, we could use the -Z minimal-versions cargo flag on the MSRV test. This would ensure that there is at least one usable set of versions that compile on MSRV.

Testing this showed that macrotest's serde dependency is too lax and should be set to 1.0.105 to work with duplicate.
eupn/macrotest#70 fixes this.

[Emoun]

Implemented awaiting next release