Python 3.12 migration and Python 3.11 by default

With the Python 3.12 release approaching, we have already started the rebuild of packages for it. Although, there is no offical Python 3.12 release yet, the release candidates of it will have the same ABI. Thus packages built with the release candidate can be safely used with the later offical release. To support rebuilding packages on conda-forge while ensuring Python release candidates don't end up in end-user solves, we have uploaded the Python 3.12.0rc2 and rc3 builds to the conda-forge/label/python_rc channel. The python312 migration adds this channel in the feedstock builds to the Python 3.12 matrix entry. On the offical release of Python 3.12, we will adjust the migration and remove the channel again. Then (on a rerender), feedstock will only consume the main channel again.

Overall, this approach allows us to provide Python 3.12 for a wide range of packages already on the day of the offical Python 3.12 release. At the same time, we have stopped the Python 3.11 migration and added it to the list of default Python versions on conda-forge.