fix: Fix to delete cookie when AppSubURL is non-empty (#30375) (#30468)

Backport #30375 by @jtran

Cookies may exist on "/subpath" and "/subpath/" for some legacy reasons
(eg: changed CookiePath behavior in code). The legacy cookie should be
removed correctly.

Co-authored-by: Jonathan Tran <jonnytran@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Kyle D <kdumontnu@gmail.com>

modules/session/store.go

@@ -6,6 +6,9 @@ package session
 import (
 	"net/http"
 
+	"code.gitea.io/gitea/modules/setting"
+	"code.gitea.io/gitea/modules/web/middleware"
+
 	"gitea.com/go-chi/session"
 )
 
@@ -18,6 +21,10 @@ type Store interface {
 
 // RegenerateSession regenerates the underlying session and returns the new store
 func RegenerateSession(resp http.ResponseWriter, req *http.Request) (Store, error) {
+	// Ensure that a cookie with a trailing slash does not take precedence over
+	// the cookie written by the middleware.
+	middleware.DeleteLegacySiteCookie(resp, setting.SessionConfig.CookieName)
+
 	s, err := session.RegenerateSession(resp, req)
 	return s, err
 }