BREAKING CHANGE: The `pdf` XBlock is now built into the platform core and
installed by default. If you previously installed a third-party pdf
implementation such as https://github.com/open-craft/xblock-pdf, then the
built-in implementation will likely work as a drop-in replacement, so you can
uninstall the third-party implementation. However, if you’d rather continue
using a third-party pdf implementation, then use the `xblock.v1.overrides`
entrypoint. In either case, the third-party implementation must be removed from
the `xblock.v1` entrypoint, otherwise you will see an AmbiguousPluginError.
Co-Authored-By: Kyle McCormick <kyle@axim.org>
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