godoc/dl: bump minimum Windows version to XP SP 3

That's what our main XP developer has, and what our builder is.

We can't vouch for anything older. Also, IIRC, there were some
TLS/cert changes in SP3 we depended on, but I can't find the details
in quick searches.

Change-Id: I876494b2c8d201e2d1a8a74ffc6150697208af6f
Reviewed-on: https://go-review.googlesource.com/83096
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>

godoc/dl/dl.go

@@ -159,7 +159,7 @@ type Feature struct {
 var featuredFiles = []Feature{
 	{
 		Platform:     "Microsoft Windows",
-		Requirements: "Windows XP SP2 or later, Intel 64-bit processor",
+		Requirements: "Windows XP SP3 or later, Intel 64-bit processor",
 		fileRE:       regexp.MustCompile(`\.windows-amd64\.msi$`),
 	},
 	{