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

src/admin/Cargo.toml

@@ -85,7 +85,7 @@ futures.workspace = true
 log.workspace = true
 ruma.workspace = true
 serde_json.workspace = true
-serde_yml.workspace = true
+serde-saphyr.workspace = true
 tokio.workspace = true
 tracing-subscriber.workspace = true
 tracing.workspace = true