*: Replace _serde with dep:serde in Cargo.toml

[GamePad64]

Remove extern crate serde everywhere else.

Description

New Cargo.toml dep: syntax removes the need of renaming serde to _serde.

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[mxinden]

I am in favor of this. @thomaseizinger is more knowledgeable when it comes to cargo features. @thomaseizinger any objections?

[thomaseizinger]

No objections, on the contrary. This is a nice simplification!

[thomaseizinger]

I think technically, this is a breaking change because we have previously provided a _serde feature. I'd ignore that though to be honest :)

[thomaseizinger]

Thank you @GamePad64 !