Fix dead_code Clippy lint with Rust 1.90+

[dzuelke]

error: struct `CaddyHTTPServerErrors` is never constructed
  --> buildpacks/static-web-server/src/caddy_config.rs:42:19
   |
42 | pub(crate) struct CaddyHTTPServerErrors {
   |                   ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: `-D dead-code` implied by `-D warnings`
   = help: to override `-D warnings` add `#[allow(dead_code)]`

GUS-W-20017598