Replace serde_yaml for registering Appservice YAML files

It is now deprecated https://github.com/dtolnay/serde-yaml/releases/tag/0.9.34

No priority because this is only used to register appservices and the entire functionality is just to parse from/to strings.

added maintenance no-priority labels

added low-priority label and removed no-priority label

May actually want to prioritise this sooner or later as someone will eventually file a rustsec advisory on serde_yaml and CI's cargo audit will start failing.

Also, yaml-rust is unmaintained and has a rustsec advisory so can't use that one: https://github.com/chyh1990/yaml-rust/issues/200

This probably isn't important to fix/do. serde_yaml is used for literally 1-2 tiny things. If someone wants to replace serde_yaml anyways a minimal PR can be accepted.

closed