Update thiserror dependency to version 2.0

[StHagel]

The thiserror crate recently upgraded to version 2.0. In my projects, log4rs is one of few crates, which still depend on thiserror version 1.0. Updating the requirement for thiserror in this crate would reduce the amount of dependencies needed.

I already tried just upgrading from 1.0.15 to 2.0 in Cargo.toml and the crate still compiled without problems and all tests passed.

[bconn98]

Awesome! Appreciate you taking time to do this. I will put it in line behind our other collection of PRs. Hoping to get them rolling again soon.