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/actions/run-python-test-set/action.yml

@@ -43,7 +43,7 @@ inputs:
   pg_version:
     description: 'Postgres version to use for tests'
     required: false
-    default: 'v14'
+    default: 'v16'
   benchmark_durations:
     description: 'benchmark durations JSON'
     required: false