publish: Move all features to feature2 if new syntax is used

[Turbo87]

Resolves #6135

[ehuss]

Seems reasonable to me.

This may change the error message users get when running cargo update in versions 1.19 to 1.51. There are different scenarios:

• The update selects a version with the new syntax, but did not need any of the features using the new syntax: Previously would get an error parsing the features.
• The update selects a version with the new syntax, and it needed a feature with the new syntax: Previously would get a failed to select a version (after 1.26? saying it "does not have these features").

With this change, I think it will always be the second case.

[bors]

☔ The latest upstream changes (presumably 8ca037e) made this pull request unmergeable. Please resolve the merge conflicts.