all: use Go 1.26 things, run most gofix modernizers

I omitted a lot of the min/max modernizers because they didn't
result in more clear code.

Some of it's older "for x := range 123".

Also: errors.AsType, any, fmt.Appendf, etc.

Updates #18682

Change-Id: I83a451577f33877f962766a5b65ce86f7696471c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/cmdname.go

@@ -39,7 +39,7 @@ func cmdName(exe string) string {
 	}
 	// v is like:
 	// "path\ttailscale.com/cmd/tailscale\nmod\ttailscale.com\t(devel)\t\ndep\tgithub.com/apenwarr/fixconsole\tv0.0.0-20191012055117-5a9f6489cc29\th1:muXWUcay7DDy1/hEQWrYlBy+g0EuwT70sBHg65SeUc4=\ndep\tgithub....
-	for _, line := range strings.Split(info, "\n") {
+	for line := range strings.SplitSeq(info, "\n") {
 		if goPkg, ok := strings.CutPrefix(line, "path\t"); ok { // like "tailscale.com/cmd/tailscale"
 			ret = path.Base(goPkg) // goPkg is always forward slashes; use path, not filepath
 			break

version/version_test.go

@@ -30,7 +30,7 @@ func readAlpineTag(t *testing.T, file string) string {
 	if err != nil {
 		t.Fatal(err)
 	}
-	for _, line := range bytes.Split(f, []byte{'\n'}) {
+	for line := range bytes.SplitSeq(f, []byte{'\n'}) {
 		line = bytes.TrimSpace(line)
 		_, suf, ok := bytes.Cut(line, []byte("FROM alpine:"))
 		if !ok {