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>

ipn/ipnlocal/local.go

@@ -3687,12 +3687,7 @@ func generateInterceptTCPPortFunc(ports []uint16) func(uint16) bool {
 			f = func(p uint16) bool { return m[p] }
 		} else {
 			f = func(p uint16) bool {
-				for _, x := range ports {
-					if p == x {
-						return true
-					}
-				}
-				return false
+				return slices.Contains(ports, p)
 			}
 		}
 	}
@@ -7387,10 +7382,8 @@ var (
 // allowedAutoRoute determines if the route being added via AdvertiseRoute (the app connector featuge) should be allowed.
 func allowedAutoRoute(ipp netip.Prefix) bool {
 	// Note: blocking the addrs for globals, not solely the prefixes.
-	for _, addr := range disallowedAddrs {
-		if ipp.Addr() == addr {
-			return false
-		}
+	if slices.Contains(disallowedAddrs, ipp.Addr()) {
+		return false
 	}
 	for _, pfx := range disallowedRanges {
 		if pfx.Overlaps(ipp) {