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/core/Cargo.toml

@@ -92,7 +92,7 @@ ruma.workspace = true
 sanitize-filename.workspace = true
 serde_json.workspace = true
 serde_regex.workspace = true
-serde_yml.workspace = true
+serde-saphyr.workspace = true
 serde.workspace = true
 smallvec.workspace = true
 smallstr.workspace = true

src/core/error/mod.rs

@@ -83,7 +83,9 @@ pub enum Error {
 	#[error(transparent)]
 	TypedHeader(#[from] axum_extra::typed_header::TypedHeaderRejection),
 	#[error(transparent)]
-	Yaml(#[from] serde_yml::Error),
+	YamlDe(#[from] serde_saphyr::Error),
+	#[error(transparent)]
+	YamlSer(#[from] serde_saphyr::ser_error::Error),
 
 	// ruma/conduwuit
 	#[error("Arithmetic operation failed: {0}")]