Move admin routers from /admin to /-/admin (#32189)

Resolve #32181

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Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

tests/integration/user_test.go

@@ -97,7 +97,6 @@ func TestRenameReservedUsername(t *testing.T) {
 
 	reservedUsernames := []string{
 		// ".", "..", ".well-known", // The names are not only reserved but also invalid
-		"admin",
 		"api",
 		"assets",
 		"attachments",