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>

runtime/internal/lib/runtime/symtab.go

@@ -140,6 +140,10 @@ type Func struct {
 	opaque struct{} // unexported field to disallow conversions
 }
 
+func (f *Func) Name() string {
+	panic("todo")
+}
+
 // moduledata records information about the layout of the executable
 // image. It is written by the linker. Any changes here must be
 // matched changes to the code in cmd/link/internal/ld/symtab.go:symtab.