Update crw-theia to use nodejs-12

Che-theia is moving to use nodejs-12 in the next sprint or two

https://github.com/eclipse/che-theia/pull/808#pullrequestreview-452509908

So, crw-theia needs to do the same for 2.4.

• affected files: https://github.com/redhat-developer/codeready-workspaces-theia (eg., Jenkinsfile, build.sh, conf/*/*dockerfile )
• new base image: https://access.redhat.com/containers/?tab=tags#/registry.access.redhat.com/ubi8/nodejs-12