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>

net/netmon/state.go

@@ -812,11 +812,8 @@ func (m *Monitor) HasCGNATInterface() (bool, error) {
 		if hasCGNATInterface || !i.IsUp() || isTailscaleInterface(i.Name, pfxs) {
 			return
 		}
-		for _, pfx := range pfxs {
-			if cgnatRange.Overlaps(pfx) {
-				hasCGNATInterface = true
-				break
-			}
+		if slices.ContainsFunc(pfxs, cgnatRange.Overlaps) {
+			hasCGNATInterface = true
 		}
 	})
 	if err != nil {