Migrate existing snapshots according to the new snapshot convention

As per swh/devel/experiments/swh-db-audit#1257 (moved), a new snapshot convention has been adopted.

This impacts our current data set as per the existing origins ingested.

In the end, only the svn loaders' origins are impacted and need migration (detailed reasoning is at swh/devel/experiments/swh-db-audit#1257 (moved)).

Note: Mercurial origins' data was not impacted as its data visits were cleaned up following another issue (swh/devel/experiments/swh-db-audit#1156 (moved)).

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Migrated from T1268 (view on Phabricator)