refactor: address final review comments

- Created zcgo.go with defaultCGO_ENABLED constant (Go 1.24 pattern)
- Updated parseGoVersion to panic on unexpected input instead of fallback
- Added runtime.(*Func).Name stub to symtab.go for Go 1.21/1.22 compatibility
- Removed go1.23 build constraint from demo
- Removed internal/buildcfg package (replaced by zcgo.go)

These changes ensure the demo works across Go 1.21+ versions.

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

_demo/go/gobuild/demo.go

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