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>

logtail/logtail.go

@@ -902,8 +902,8 @@ func parseAndRemoveLogLevel(buf []byte) (level int, cleanBuf []byte) {
 	if bytes.Contains(buf, v2) {
 		return 2, bytes.ReplaceAll(buf, v2, nil)
 	}
-	if i := bytes.Index(buf, vJSON); i != -1 {
-		rest := buf[i+len(vJSON):]
+	if _, after, ok := bytes.Cut(buf, vJSON); ok {
+		rest := after
 		if len(rest) >= 2 {
 			v := rest[0]
 			if v >= '0' && v <= '9' {

logtail/logtail_test.go

@@ -86,10 +86,10 @@ func TestDrainPendingMessages(t *testing.T) {
 	}
 
 	// all of the "log line" messages usually arrive at once, but poll if needed.
-	body := ""
+	var body strings.Builder
 	for i := 0; i <= logLines; i++ {
-		body += string(<-ts.uploaded)
-		count := strings.Count(body, "log line")
+		body.WriteString(string(<-ts.uploaded))
+		count := strings.Count(body.String(), "log line")
 		if count == logLines {
 			break
 		}