api: raw extrinsic metadata endpoint api fails with python exception about missing attribute

As reported by sentry [1], webapps (staging [2], prod [3] [4]) fails with a python exception.

It seems the current deployed version of the storage (v0.31) is out of sync with what the webapp expects (at least v0.32).

Fix it.

• [1] https://sentry.softwareheritage.org/share/issue/18076271280b4eb6843241f5296c37bb/

• [2] https://webapp.staging.swh.network/api/1/raw-extrinsic-metadata/swhid/swh:1:dir:a2faa28028657859c16ff506924212b33f0e1307/authorities/

• [3] https://archive.softwareheritage.org/api/1/raw-extrinsic-metadata/swhid/swh:1:dir:a2faa28028657859c16ff506924212b33f0e1307/authorities/

• [4] https://webapp1.internal.softwareheritage.org/api/1/raw-extrinsic-metadata/swhid/swh:1:dir:a2faa28028657859c16ff506924212b33f0e1307/authorities/

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Migrated from T3436 (view on Phabricator)