use older function to support old Postgres

[vincentsarago]

closes #67

This PR replace string_to_table(...) by unnest(string_to_array(...)) to support older postgres

string_to_table() provides the set-based equivalent to string_to_array(), and is functionally the same albeit somewhat faster than unnest(string_to_array())

From PG docs: https://pgpedia.info/s/string_to_table.html

Note: we could also use regexp_split_to_table https://pgpedia.info/r/regexp_split_to_table.html which seems to give the same result 🤷

[vincentsarago]

👋 @bitner ☝️