Due to the way it was first implemented, the "username" column would be
on a different position depending on if it was an extension upgrade or
a fresh install.
This caused problems with pg_dumping databases and restoring them.
pg_dump does not include the extension source (so this table's schema is
not included on the dump) but does include this contents, using a COPY
without column names by default and failing due to the order difference.
After this has run, all tables (whether updated or not) will have the
"username" column on the last position.
