govuk_solr6: add (multi-valued) `harvest` field to ckan28 schema
govuk_solr6: add (multi-valued) `harvest` field to ckan28 schema

the exact origins of the requirement for this are unclear, but this
mirrors what was previously done in
https://github.com/alphagov/datagovuk_ckan_deployment/pull/63. without
this, our current database fails to be reindexed as we have many packages
with apparently multi-valued `harvest` fields.

alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet,alphagov/govuk-puppet