feat(demo): add go1.23 build tag to unique demo

Added //go:build go1.23 constraint to the unique demo.
This allows Go's build system to automatically skip this demo
on Go versions older than 1.23, avoiding compilation errors
where the unique package doesn't exist.

This approach is cleaner than version checking in test scripts
and leverages Go's native build tag system.

Generated with [codeagent](https://github.com/qbox/codeagent)
Co-authored-by: cpunion <cpunion@users.noreply.github.com>

_demo/go/unique/main.go

@@ -1,3 +1,5 @@
+//go:build go1.23
+
 package main
 
 import "unique"