tidy: forbid since values for features that point to the current release or future...

tidy: forbid since values for features that point to the current release or future ones

It's a common phenomenon that feature stabilizations don't make it into
a particular release, but the version is still inaccurate. Often this
leads to subsequent changes to adjust/correct the version.

Instead, require people to put a placeholder that gets replaced during
beta branching time with the current rust version. That way, there is
no chance that an error can be introduced.

Usage of the placeholder is required on the nightly channel, and forbidden
on the stable and beta channels.