CI: Bump to NodeJS 20

.gitlab-ci.yml

 # SPDX-FileCopyrightText: 2019-2022 Pleroma Authors <https://pleroma.social>
 # SPDX-License-Identifier: AGPL-3.0-only
 
-image: node:16-alpine
+image: node:20-alpine
 
 variables: &global_variables
   DOCKER_DRIVER: overlay2