Add robots.txt as reserved username (#6272)
Add robots.txt as reserved username (#6272)

Fix #6271
kubatyszko/gitea,lafriks/gitea,vdbt/gitea,lafriks/gitea,go-gitea/gitea,vCaesar/gitea,vdbt/gitea,sapk-fork/gitea,lafriks/gitea,vCaesar/gitea,kubatyszko/gitea,typeless/gitea,typeless/gitea,sapk-fork/gitea,cez81/gitea,typeless/gitea,sapk-fork/gitea,typeless/gitea,cez81/gitea,sapk-fork/gitea,kubatyszko/gitea,go-gitea/gitea,cez81/gitea,go-gitea/gitea