refactor(engine): use Rust edition 2021

This change is because of a rustfmt bug in the 2024 edition.

For example, the following code snippet:
```
struct Value {}
struct ParsingError {}
trait Keyword {}

impl Value {
    fn parse<KeywordT: Keyword>(value: &str, line_no: usize) -> Result<Self, ParsingError> {
        todo!();
    }
}
```

when formatted with `rustfmt --edition 2021 --emit stdout --config max_width=90,brace_style=AlwaysNextLine` becomes:
```
struct Value {}
struct ParsingError {}
trait Keyword {}

impl Value
{
    fn parse<KeywordT: Keyword>(value: &str, line_no: usize)
    -> Result<Self, ParsingError>
    {
        todo!();
    }
}
```

0 files changed, 0 insertions, 0 deletions