The original migration to import the participants for public OCMW associations
contained a bug causing it to not match municipalities. More specifically:
- the type triple for municipalities is located in the `shared` graph;
- consequently the `FILTER EXISTS` statements to limit matches to administrative
units could not succeed for municipalities as the queries are limited to the
`administrative-unit` graph.
This new migration fixes this problem by changing the `FILTER EXISTS` into a
`FILTER NOT EXISTS` to exclude resourses of type `ext:KboOrganisatie`.
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