Return 404 instead of 403 if user can not access the repo (#23155) (#23158)

Backport #23155

Fixes #23150

Before:

![image](https://user-images.githubusercontent.com/18380374/221390802-2317c6bc-d163-4def-b68b-6bb297143fe2.png)

After:

![image](https://user-images.githubusercontent.com/18380374/221390823-87490351-39c3-4a40-b1d2-11fc5b85fa24.png)

Co-authored-by: yp05327 <576951401@qq.com>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

Author: 3 people

routers/web/repo/http.go

@@ -228,7 +228,7 @@ func httpBase(ctx *context.Context) (h *serviceHandler) {
 				}
 
 				if !p.CanAccess(accessMode, unitType) {
-					ctx.PlainText(http.StatusForbidden, "User permission denied")
+					ctx.PlainText(http.StatusNotFound, "Repository not found")
 					return
 				}
 			}