fix: add Go 1.23+ build constraint to gobuild demo

Add //go:build go1.23 constraint to skip compilation on Go 1.21/1.22
where runtime.(*Func).Name is not implemented in llgo, causing linker
errors when internal/bisect is pulled in as a dependency.

The demo works correctly on Go 1.23+ where the dependency chain or
internal/bisect behavior avoids calling the unimplemented method.

Fixes compatibility issue reported in PR review.

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

Author: xgopilot

_demo/go/gobuild/demo.go

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