chore!: always use async walredo, warn if sync is configured (#7754)

refs https://github.com/neondatabase/neon/issues/7753

This PR is step (1) of removing sync walredo from Pageserver.

Changes:
* Remove the sync impl
* If sync is configured, warn! and use async instead
* Remove the metric that exposes `kind`
* Remove the tenant status API that exposes `kind`

Future Work
-----------

After we've released this change to prod and are sure we won't
roll back, we will

1. update the prod Ansible to remove the config flag from the prod
   pageserver.toml.
2. remove the remaining `kind` code in pageserver

These two changes need no release inbetween.

See  https://github.com/neondatabase/neon/issues/7753 for details.

libs/pageserver_api/src/models.rs

@@ -776,9 +776,6 @@ pub struct TimelineGcRequest {
 #[derive(Debug, Clone, Serialize, Deserialize)]
 pub struct WalRedoManagerProcessStatus {
     pub pid: u32,
-    /// The strum-generated `into::<&'static str>()` for `pageserver::walredo::ProcessKind`.
-    /// `ProcessKind` are a transitory thing, so, they have no enum representation in `pageserver_api`.
-    pub kind: Cow<'static, str>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize)]