Update python version to 3.12 and base to core24 in snaps (#9983)

Fixes #9872, originally merged in #9956.

To upgrade to python3.12 as 3.8 is reaching EOL, we need to upgrade the core snap that certbot is based on. The latest version is core24, so we're going with that for longevity. We will want to notify third party snaps to make changes as well. They can release their snaps to a version higher than certbot's, and their users will not be upgraded until the matching (or greater) version of certbot is released. They should do this as otherwise including these changes will break their plugins.

Key documents for this migration are https://snapcraft.io/docs/migrate-core22 and https://snapcraft.io/docs/migrate-core24. The discussion at https://forum.snapcraft.io/t/upgrading-classic-snap-to-core24-using-snapcraft-8-3-causes-python-3-12-errors-at-runtime/ is also relevant to understanding some changes, which may become unnecessary in future versions of snapcraft.


* Migrate primary certbot snap to core24 and python 3.12

* Migrate plugin snaps to core24 and python 3.12

* Migrate to core24 in build_remote

* Run snap tests using python 3.12

* Unstage pyvenv.cfg and set PYTHONPATH

---------

Co-authored-by: Erica Portnoy <ebportnoy@gmail.com>
Co-authored-by: Erica Portnoy <erica@eff.org>

certbot/docs/contributing.rst

@@ -376,8 +376,8 @@ Certbot plugin snaps expose their Python modules to the Certbot snap via a
 `snap content interface`_ where ``certbot-1`` is the value for the ``content``
 attribute. The Certbot snap only uses this to find the names of connected
 plugin snaps and it expects to find the Python modules to be loaded under
-``lib/python3.8/site-packages/`` in the plugin snap. This location is the
-default when using the ``core20`` `base snap`_ and the `python snapcraft
+``lib/python3.12/site-packages/`` in the plugin snap. This location is the
+default when using the ``core24`` `base snap`_ and the `python snapcraft
 plugin`_.
 
 The Certbot snap also provides a separate content interface which