The override added in a750937 had no effect: _load_meta only copies a fixed
key whitelist into the meta dict, and UPGRADE_BASE_VERSION wasn't in it, so
meta.get(...) returned None and the upgrade base fell back to previous_version()
= recipe_versions[-2] (0.6.3+3.1.2). Add it to the whitelist so discourse's
honest 0.7.0 base is selected.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>