Update node versions used in Dockerfile

[szschaler]

The central Dockerfile currently uses Node 19 in both the frontend static-build stages (node:19-bullseye base image at stages staticbuild-emf, staticbuild-emfatic, staticbuild-ocl, staticbuild-conversion, staticbuild-xtext) and in the Xtext service stage (NODE_VERSION=19.9.0). Node 19 reached end-of-life and is no longer receiving security patches.

Tasks:

• Replace node:19-bullseye in all five frontend static-build stages with the current LTS base image (e.g. node:20-bullseye or node:22-bullseye)
• Update ENV NODE_VERSION=19.9.0 in the Xtext service stage to a currently supported LTS release
• Keep all Node versions consistent across stages
• Verify the npm builds still pass after the upgrade

[szschaler]

Extracted from #111