Change Rust edition to 2024 from 2021 for "dx new app"

I just realized that the default Rust edition in Cargo.toml file is still 2021 for dx creating a new project.
It's 2026 now, lots of Rust crates are moving to edition 2024 so that we can benefit all the improvement directly.

It's just a proposal, and I would like to hear your feedback on decision. Thanks a lot! 

