Use core::error::Error in codebase

[tyranron]

Synopsis

We had TODOs across the code:

// TODO: Use `derive_more::core::error::Error` once `error_in_core` Rust feature is
//       stabilized.

The feature(error_in_core) was stabilized in 1.81 Rust.
It's long enough, so we can bump up our MSRV.

Solution

• Use core::error::Error in codebase and remove feature = "std" gating for it.
• Bump up MSRV to 1.81.

Checklist

• Documentation is updated
• Tests are added/updated
• CHANGELOG entry is added

[tyranron]

@JelteF please, update also required CI tasks to MSRV 1.81.0. Thanks!

[tyranron]

@JelteF to merge this we require to change required CI jobs to 1.81.0 MSRV in master branch protection rules.

[JelteF]

@JelteF to merge this we require to change required CI jobs to 1.81.0 MSRV in master branch protection rules.

I think it would be good to remove the version number from the MSRV build name. So we don't have this problem everytime we bump it.

[tyranron]

@JelteF done!