Replace httr with httr2

[Bisaloo]

The deprecation of httr is moving faster than what we anticipated. All pending PR requests have been closed by the following message:

httr has been superseded in favour of httr2, so is no longer under active development. Thanks for using httr, thanks for contributing, and my apologies that your contribution never made it into the package.

To ensure sustainability of readepi, we should replace httr to httr2 at some point. This is however not a priority for now and should wait until the first release of readepi is live.