Update node minimum version constraint to node 14

[tortmayr]

Node 12 as reached EOL and is no longer actively supported with security updates. Some project e.g the vscode integration can already no longer be built using node 12. For Theia a PR to drop node 12 support is pending (https://github.com/eclipse-theia/theia/pull/11210).

We should update the doc and engine constraints in all repositories to list Node 14 as new minimum version. https://github.com/eclipse-glsp/glsp/discussions/671#discussioncomment-2872988

[planger]

Let's also add the node version constraint to the https://github.com/eclipse-glsp/glsp-examples/tree/master/project-templates. Once this is done, we should move the tag https://github.com/eclipse-glsp/glsp-examples/tree/generator-latest/project-templates to include this change in the generator extension.