Make the captcha shorter

services/context/captcha.go

@@ -5,6 +5,7 @@ package context
 
 import (
 	"fmt"
+	"math/rand"
 	"sync"
 
 	"forgejo.org/modules/base"
@@ -55,7 +56,9 @@ func GetImageCaptcha() string {
 	imageCaptchaOnce.Do(func() {
 		captcha.SetCustomStore(&imageCaptchaStore{c: cache.GetCache()})
 	})
-	return captcha.New()
+
+	length := 4 + rand.Intn(2)
+	return captcha.NewLen(length)
 }
 
 // SetCaptchaData sets common captcha data