chore: replace serde-yml with serde-saphyr

- serde-yml has an un-addressed [security issue][sec-issue]
- [saphyr][saphyr] is a pretty recent and active crate that deals with YAML parsing
- based on that, someone recently created [serde-saphyr][serde-saphyr]

---

The change was pretty straightforward and mostly "just a search and replace". The new crate has it's `Error` type split
into serialization and derserialization errors. Hence I created one Continuwuity-Error variant for each instead of just
having a single `Yaml` variant. This was already done previously with the `Toml` errors so I thought this would be
rather acceptable.

[sec-issue]: https://github.com/advisories/GHSA-gfxp-f68g-8x78
[saphyr]: https://github.com/saphyr-rs/saphyr
[serde-saphyr]: https://github.com/saphyr-rs/saphyr/issues/66#issuecomment-3353212289

Cargo.toml

@@ -166,8 +166,8 @@ default-features = false
 features = ["raw_value"]
 
 # Used for appservice registration files
-[workspace.dependencies.serde_yml]
-version = "0.0.12"
+[workspace.dependencies.serde-saphyr]
+version = "0.0.7"
 
 # Used to load forbidden room/user regex from config
 [workspace.dependencies.serde_regex]