Allow cargo-generate 0.21.

[abizjak]

Purpose

Fixes #429

Changes

Allow cargo generate 0.21. The changes from 0.20.* are not important https://github.com/cargo-generate/cargo-generate/releases/tag/v0.21.0

Checklist

• [x] My code follows the style of this project.
• [x] The code compiles without warnings.
• [x] I have performed a self-review of the changes.
• [x] I have documented my code, in particular the intent of the hard-to-understand areas.
• [x] (If necessary) I have updated the CHANGELOG.

[abizjak]

@DOBEN I think you need to merge this when ready since I should not longer have write access.