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/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 {