Mark the built-in blocks deprecated

[farhan]

• Update enabled extracted block feature flag docs
• Add deprecated warning to the enabled built-in blocks

[kdmccormick]

thanks for opening this!

[kdmccormick]

Perfect 💯

I tested the warnings with built-in LTI and extracted HTML:

In [1]: from xblock.core import XBlock ; XBlock.load_class("lti")
2026-03-17 14:36:48,443 WARNING 39 [py.warnings] [user None] [ip None] warnings.py:112 - /opt/pyenv/versions/3.12.12/lib/python3.12/importlib/__init__.py:90: DeprecationWarning: The built-in `xmodule.lti_block` LTIBlock implementation is deprecated. To fix this warning, enable `USE_EXTRACTED_LTI_BLOCK` (set it to True) to use `xblocks_contrib.lti.LTIBlock` instead. Support for the built-in implementation, and the `USE_EXTRACTED_LTI_BLOCK` setting, will be removed in Willow.
  return _bootstrap._gcd_import(name[level:], package, level)
Out[1]: xmodule.lti_block._BuiltInLTIBlock

In [2]: from xblock.core import XBlock ; XBlock.load_class("html")
Out[2]: xblocks_contrib.html.html.HtmlBlock