Switch from deprecated serde_yaml to supported fork serde_yaml_ng

[lucas42]

The widely used serde_yaml package is deprecated and no longer maintained.

A fork which is still maintained is serde_yaml_ng, which promises backwards compatibility with the old library.

Caveats

• The topic of which yaml library to move to is still widely debated in the rust community, eg: https://users.rust-lang.org/t/serde-yaml-deprecation-alternatives/108868
• The library I've picked has 80 stars on github, which is a decent number, but pales into comparison of the 1000 stars of the previous library
• I'm very new to writing rust (only really started yesterday), so don't take my word on the best way forward.