fix: Avoid useless reference usage in user_ldap

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

apps/user_ldap/lib/Group_Proxy.php

 			$this->backends[$configPrefix] =
 				new Group_LDAP($this->getAccess($configPrefix), $this->groupPluginManager, $this->config, $this->ncUserManager);
 			if (is_null($this->refBackend)) {
-				$this->refBackend = &$this->backends[$configPrefix];
+				$this->refBackend = $this->backends[$configPrefix];
 			}
 		}
 

apps/user_ldap/lib/LDAP.php

 			$serverControls = [];
 		}
 
+		/** @psalm-suppress UndefinedVariable $oldHandler is defined when the closure is called but psalm fails to get that */
 		$oldHandler = set_error_handler(function ($no, $message, $file, $line) use (&$oldHandler) {
 			if (str_contains($message, 'Partial search results returned: Sizelimit exceeded')) {
 				return true;

apps/user_ldap/lib/User_Proxy.php

 			);
 
 			if (is_null($this->refBackend)) {
-				$this->refBackend = &$this->backends[$configPrefix];
+				$this->refBackend = $this->backends[$configPrefix];
 			}
 		}