migrate to this error

it doesn't generate a bazillion warning at compile time

src/api/k2v/Cargo.toml

@@ -20,7 +20,7 @@ garage_util = { workspace = true, features = [ "k2v" ] }
 garage_api_common.workspace = true
 
 base64.workspace = true
-err-derive.workspace = true
+thiserror.workspace = true
 tracing.workspace = true
 
 futures.workspace = true