Before this change during downgrade script generation we would
always fetch the pre-update script from the previous version and
prepend it to the generated scripts. This limits what can be
referenced in the pre-update script and also what is possible
within the downgrade itself.
This patch splits the pre-update script into a generic part that
is used for update/downgrade and an update specific part. We could
later also add a downgrade specific part but currently it is not
needed. This change is necessary because we reference a timescaledb
view in the pre-update script which prevents changes to that view.
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