Use better query for availability.

In #305, the query added in PR #1550 (commit 28b149f) uses PostgreSQL's `range_agg()`, which did not become available until version 14.  EL9 ships with 13 and EL8 ships with 10.

Once all distributions support 14, switching back to the code in the original commit.  It's a better query and will probably run a bit faster.
