Migrate to v4 of Azure Functions runtime

[danuker]

Azure Functions v3 which we use is no longer supported and we should migrate to v4.

I am not sure if it's worth it, however, since we plan to switch to a GitHub app (#29). But if it's easy to do (since MS claims good backwards compatibility), we should.

[adiroiban]

If it's easier for you, we can forget about the whole serverless part.

What we can do is pack the application in a minimal container and deploy it at first in our ES DC.

And if all is ok, we can deploy it on Azure via something like https://azure.microsoft.com/en-gb/products/container-apps/

or forget about the while "serverless" framework and write the application directly for Azure Functions.

The reason why I wanted to try "the serverless technology" and see if we can use it later for "serious business"