feat!: Switch v2 libraries to Learning Core data models (#34066)

This moves the Content Libraries V2 backend from Blockstore [1] over to
Learning Core [2] For high-level overview and rationale of this move, see
the Blockstore DEPR [3]. There are several follow-up tasks [4], most notably
adding support for static assets in libraries.

BREAKING CHANGE: Existing V2 libraries, backed by Blockstore, will stop
working. They will continue to be listed in Studio, but their content
will be unavailable. They need to be deleted (via Django admin) or manually
migrated to Learning Core. We do not expect production sites to be in
this situation, as the feature has never left "experimental" status.

[1] https://github.com/openedx-unsupported/blockstore
[2] https://github.com/openedx/openedx-learning/
[3] https://github.com/openedx/public-engineering/issues/238
[4] https://github.com/openedx/edx-platform/issues/34283

openedx/core/lib/blockstore_api/__init__.py

@@ -1,9 +1,7 @@
 """
 API Client for Blockstore
 
-This API does not do any caching; consider using BundleCache or (in
-openedx.core.djangolib.blockstore_cache) together with these API methods for
-improved performance.
+TODO: This should all get ripped out.
 
 TODO: This wrapper is extraneous now that Blockstore-as-a-service isn't supported.
       This whole directory tree should be removed by https://github.com/openedx/blockstore/issues/296.

openedx/core/lib/xblock_serializer/api.py

@@ -23,5 +23,10 @@ def serialize_modulestore_block_for_blockstore(block):
             contain the OLX of its children, just refers to them, so you have to
             separately serialize them.)
         (3) a list of any static files required by the XBlock and their URL
+
+    TODO: We should deprecate this in favor of a new Learning Core implementation.
+    We've left it as-is for now partly because there are bigger questions that
+    we have around how we should rewrite this (e.g. are we going to
+    remove <xblock-include>?).
     """
     return XBlockSerializerForBlockstore(block)