Support Rust 2021 edition

[TonalidadeHidrica]

The README says that this tool supports Rust Edition 2018, not 2015. But how about 2021? It's already been two years since the release, and many competitive programming websites including AtCoder now adopt it. I wonder what is the obstacles for supporting the new editiion. (At least, I recall it works for 2021 edition as long as used normally, although it's been a while since I used this tool last time.)

[mizar]

It is already working in Rust 2021, but I guess they just haven't modified the README.

• https://github.com/qryxip/cargo-equip/blob/master/CHANGELOG.md#0200---2023-08-06z
• cargo-equip Incompatible minify results with Rust 2021 #194
• Bump rustminify to v0.2.0 #198
• Incompatible minify results with Rust 2021 rustminify#6
• Prevent minify from generating reserved prefixes in Rust 2021 edition rustminify#7