Use `httr2` instead of `httr`

[fh-mthomson]

Feature requests related to interacting with the Connect API via R: migrate from httr -> httr2 for each:

1. connectapi source code
2. R examples in Connect API Reference

Goals:

1. Drive adoption of httr2 (over httr)
2. Ensure users trying to use httr2 are doing so in a secure way - https://github.com/r-lib/httr2/issues/420#issue-2051486433