GH-348: [Release] Update versions for 16.0.0 (#357)

Closes https://github.com/apache/arrow-cookbook/issues/348

This PR:

- Updates versions from 15.0.0 to 16.0.0
- Updates the R cookbook to pass under 16.0.0
- Updates the C++ cookbook to pass under 16.0.0
- Changes `test_cpp_cookbook.yml` to now always test against stable libarrow.
  This is a workaround for not having an up-to-date
  conda-nightlies channel, see
  https://github.com/apache/arrow/issues/41856
- Adds a note in the top-level README about the status of the dev cookbooks
12 files changed