fix: Use edition 2024 by default

[yutannihilation]

This supersedes #3.

The problem is that, while this template uses edition 2021, cargo generate generates a crate with the default edition of the toolchain. So, this compile failure is what a user would see if they uses the latest stable version of Rust.

error: unsafe attribute used without unsafe
  --> src\main.rs:77:3
   |
77 | #[link_section = ".bi_entries"]
   |   ^^^^^^^^^^^^ usage of unsafe attribute
   |
help: wrap the attribute in `unsafe(...)`
   |
77 | #[unsafe(link_section = ".bi_entries")]
   |   +++++++                            +

error: could not compile `rp235x-project-template` (bin "rp235x-project-template") due to 1 previous error

Marking the attribute in unsafe(...) doesn't require Rust 2024 edition (this syntax is valid since Rust 1.82 as suggested in #3 (comment)), but, since this repository is tested against the stable and nightly version, I think it's a good idea to test with the default edition of such toolchains.

[9names]

LGTM, thanks!
Closes #3