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>

wgengine/filter/filter_test.go

@@ -751,13 +751,13 @@ func ports(s string) PortRange {
 	}
 
 	var fs, ls string
-	i := strings.IndexByte(s, '-')
-	if i == -1 {
+	before, after, ok := strings.Cut(s, "-")
+	if !ok {
 		fs = s
 		ls = fs
 	} else {
-		fs = s[:i]
-		ls = s[i+1:]
+		fs = before
+		ls = after
 	}
 	first, err := strconv.ParseInt(fs, 10, 16)
 	if err != nil {