Set 1.75 as minimal Rust version (#11821)
## Summary
Salsa requires https://github.com/rust-lang/rust/issues/91611 which
stabalized with
[1.75](https://blog.rust-lang.org/2023/12/28/Rust-1.75.0.html)
1.75 doesn't change the supported platforms. So I think this upgrade is
fine in a minor.
## Test Plan
`cargo build`
