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/serve.go

@@ -835,8 +835,8 @@ func (b *LocalBackend) proxyHandlerForBackend(backend string) (http.Handler, err
 	targetURL, insecure := expandProxyArg(backend)
 
 	// Handle unix: scheme specially
-	if strings.HasPrefix(targetURL, "unix:") {
-		socketPath := strings.TrimPrefix(targetURL, "unix:")
+	if after, ok := strings.CutPrefix(targetURL, "unix:"); ok {
+		socketPath := after
 		if socketPath == "" {
 			return nil, fmt.Errorf("empty unix socket path")
 		}