Support pg17

[thanodnl]

There are no functional changes required for t-digest in pg17, however, test output has changed ever so slightly in the form of

--- /workspaces/tdigest/test/expected/parallel_query.out	2024-08-30 14:04:57.324801940 +0000
+++ /workspaces/tdigest/results/parallel_query.out	2024-09-19 13:36:15.678544507 +0000
@@ -48,7 +48,7 @@
 ------------------------------------------------------
  Subquery Scan on foo
    ->  Finalize Aggregate
-         InitPlan 1 (returns $1)
+         InitPlan 1
            ->  Aggregate
                  ->  Gather
                        Workers Planned: 2

To accommodate for these changes we have added two alternative output files that will make the tests pass.

To ensure compatibility we add pg17 to CI as well. Fixes: #31