Rename 'ReturnUrl' to 'returnUrl' for forms auth redirection

(cherry picked from commit 812712e2843a738054c065a6d5c1b7c81c5f8e7b)
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f