Fix `incompatible_msrv_exceptions` clippy warnings

[spacebear21]

We're ignoring potential MSRV incompatibilities in payjoin-cli with #[allow(clippy::incompatible_msrv_exceptions)]. These should be removed and fixed.

Gotta open an issue to fix these incompatible_msrv exceptions. They're not MSRV compliant!

Originally posted by @DanGould in #750 (comment)

[0xZaddyy]

Hey man, would bumping the MSRV to 1.64.0 be an acceptable solution here?
i can see it's one of the cleanest solution.

[spacebear21]

We must stay on 1.63 to respect the MSRV in the wider rust bitcoin ecosystem (e.g. rust-bitcoin, bdk, rust-lightning).

[shinghim]

It looks like this lint was added in 1.78