Update MSRV

[jamillambert]

The decision at the Rust Bitcoin Summit was to have an MSRV in corepc of 1.75.0.

rust-bitcoin is currently at 1.74.0 but there is a change in 1.75.0 that is relevant to corepc mentioned at the summit, I think it is:

• Stabilize async fn and return-position impl Trait in traits.

Opening the issue to confirm this is correct before doing it.

[tcharding]

I can't remember exactly why 1.75 but that is the version I think we use for this crate. Lets go.