Update maturin build backend init template (#16449)
## Summary
Upgrades to the latest Rust edition and pyo3 version.
Change initialized module to "Inline Declaration" format.
https://pyo3.rs/v0.27.1/module.html#inline-declaration
The output of `maturin new` is also updating to the new declarative
format
https://github.com/PyO3/maturin/commit/342239a95a0f3274ad3224e835dddfdb08737e15
## Test Plan
Updated the relevant snapshot tests and to confirm ran
`cargo nextest run --all-features --no-fail-fast maturin`
Also used the updated template to generate a library in a different
project with
```
cargo run -- init --lib --build-backend maturin --name try-template ../_OTHER_PROJECT_/backend/try-template
```
and the generated workspace member functioned as expected.
