Metadata search is failing with "failed to parse date field"

Any alphanumerical metadata search fails with swh-search, a numerical only term seems to work. The exception is :

RequestError(400, 'search_phase_execution_exception', 'failed to parse date field [blah] with format [strict_date_optional_time||epoch_millis]: [failed to parse date field [blah] with format [strict_date_optional_time||epoch_millis]]')

Link to the exception on sentry: https://sentry.softwareheritage.org/share/issue/2cb7ca683aa64ac490512676d72a3d63/

(it's only deployed on staging and on webapp1 currently. For webapp1, the index is not populated with the metadata so it's not failing)

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Migrated from T3058 (view on Phabricator)