set-version: add support for workspace version

[MarcoIeni]

Motivations

It would be nice to change the workspace version via release-plz.

However, at the moment release-plz set-version 1.2.3 only works in projects with a single crate (not in a workspace).

Solution

release-plz set-version <version> is also allowed in workspaces that have a workspace version.
There's already a test written in #1552 (I don't plan to work on that PR, so if you want to see this happen, you can write it here and start working on it).

You should update all the changelogs of all the packages that inherit the workspace version 👍