Consider an appropriate MSRV

[elpiel]

Since we are moving forward with implementations that will bring async support I think it's good time to talk about MSRV.

async_fn_in_trait has been stabalised in 1.75.0 and I think we can start from there.
I can:

• tweak the CI to include MSRV build
  as well as
• change the Cargo.toml to include rust-version = "1.75"

[thejpster]

I'm fine with either 1.75, or Ferrocene 24.05 (which is based off 1.76).