Make Postgres 16 default version (#8745)

## Problem

The default Postgres version is set to 15 in code, while we use 16 in
most of the other places (and Postgres 17 is coming)

## Summary of changes
- Run `benchmarks` job with Postgres 16 (instead of Postgres 14)
- Set `DEFAULT_PG_VERSION` to 16 in all places
- Remove deprecated `--pg-version` pytest argument
- Update `test_metadata_bincode_serde_ensure_roundtrip` for Postgres 16

.github/workflows/build_and_test.yml

@@ -280,6 +280,7 @@ jobs:
           save_perf_report: ${{ github.ref_name == 'main' }}
           extra_params: --splits 5 --group ${{ matrix.pytest_split_group }}
           benchmark_durations: ${{ needs.get-benchmarks-durations.outputs.json }}
+          pg_version: v16
         env:
           VIP_VAP_ACCESS_TOKEN: "${{ secrets.VIP_VAP_ACCESS_TOKEN }}"
           PERF_TEST_RESULT_CONNSTR: "${{ secrets.PERF_TEST_RESULT_CONNSTR }}"