net: use internal/itoa for Go < 1.26 to fix build with older Go versions

internal/strconv was added in Go 1.26. For older Go versions, use
internal/itoa (already present in TinyGo) via a build tag shim.

Fixes TinyGo #5332

Signed-off-by: deadprogram <ron@hybridgroup.com>

itoa_go126.go

@@ -0,0 +1,7 @@
+//go:build go1.26
+
+package net
+
+import "internal/strconv"
+
+func netItoa(i int) string { return strconv.Itoa(i) }