chore(pmtud): remove calls to syscall in favor of unix and windows

- syscall is deprecated and is not kept up-to-date
- each OS is inherently different hence the syscall being deprecated

internal/pmtud/ip/ipv6_windows.go

@@ -1,7 +1,7 @@
 package ip
 
-import "syscall"
+import "golang.org/x/sys/windows"
 
-func SetIPv6HeaderIncluded(fd syscall.Handle) error {
+func SetIPv6HeaderIncluded(fd windows.Handle) error {
 	panic("windows does not allow an application to build IPv6 headers")
 }