Make esap-gui fully compatible with modern Node

The node:lts Docker image recently switched from providing Node v16.18 to v18.12. Our build broke as a result.

One https://git.astron.nl/astron-sdc/esap-gui/-/merge_requests/105 we've put in a temporary fix to unbreak the build, but in practice we ought to revert that hack and do the upgrade “properly”.