chore: update Go dependencies (#2540)
Update Go module dependencies in the root and `caddy/` modules.
## Changes
- `prometheus/client_golang` v1.23.2 → v1.24.0 (both modules)
- `klauspost/compress` v1.19.0 (caddy indirect); pruned from root
indirect by `go mod tidy`
## Verification
- `go mod tidy` + `go mod verify`: all modules verified.
- Root non-cgo tests: 792 passed. cgo packages skipped locally (no
PHP/brotli headers).
