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

src/admin/appservice/commands.rs

@@ -16,7 +16,7 @@ pub(super) async fn register(&self) -> Result {
 
 	let range = 1..checked!(body_len - 1)?;
 	let appservice_config_body = body[range].join("\n");
-	let parsed_config = serde_yml::from_str(&appservice_config_body);
+	let parsed_config = serde_saphyr::from_str(&appservice_config_body);
 	match parsed_config {
 		| Err(e) => return Err!("Could not parse appservice config as YAML: {e}"),
 		| Ok(registration) => match self
@@ -57,7 +57,7 @@ pub(super) async fn show_appservice_config(&self, appservice_identifier: String)
 	{
 		| None => return Err!("Appservice does not exist."),
 		| Some(config) => {
-			let config_str = serde_yml::to_string(&config)?;
+			let config_str = serde_saphyr::to_string(&config)?;
 			write!(self, "Config for {appservice_identifier}:\n\n```yaml\n{config_str}\n```")
 		},
 	}