modernize delta-notifier

[nvdk]

• move to node 18
• set up body parser in line with doc of javascript template
• removed usage of request, since fetch is now native under node 18
• set up keep alive to reuse tcp sockets

~NOTE: uses nvdk/mu-javascript-template since the semtech one doesn't have recent build~

[nvdk]

tested in two stacks (app-vocabsearch and app-octopus), pretty much a drop in replacement as far as I can tell