Clean up 'attachment service' names to storage controller (#7326)

The binary etc were renamed some time ago, but the path in the source
tree remained "attachment_service" to avoid disruption to ongoing PRs.
There aren't any big PRs out right now, so it's a good time to cut over.

- Rename `attachment_service` to `storage_controller`
- Move it to the top level for symmetry with `storage_broker` & to avoid
mixing the non-prod neon_local stuff (`control_plane/`) with the storage
controller which is a production component.

diesel.toml

@@ -2,8 +2,8 @@
 # see https://diesel.rs/guides/configuring-diesel-cli
 
 [print_schema]
-file = "control_plane/attachment_service/src/schema.rs"
+file = "storage_controller/src/schema.rs"
 custom_type_derives = ["diesel::query_builder::QueryId"]
 
 [migrations_directory]
-dir = "control_plane/attachment_service/migrations"
+dir = "storage_controller/migrations"