Feat/postgres 16 (#4761)

This adds PostgreSQL 16 as a vendored postgresql version, and adapts the
code to support this version. 
The important changes to PostgreSQL 16 compared to the PostgreSQL 15
changeset include the addition of a neon_rmgr instead of altering Postgres's
original WAL format.

Co-authored-by: Alexander Bayandin <alexander@neon.tech>
Co-authored-by: Heikki Linnakangas <heikki@neon.tech>

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

@@ -70,6 +70,9 @@ runs:
         name: compatibility-snapshot-${{ inputs.build_type }}-pg${{ inputs.pg_version }}
         path: /tmp/compatibility_snapshot_pg${{ inputs.pg_version }}
         prefix: latest
+        # The lack of compatibility snapshot (for example, for the new Postgres version)
+        # shouldn't fail the whole job. Only relevant test should fail.
+        skip-if-does-not-exist: true
 
     - name: Checkout
       if: inputs.needs_postgres_source == 'true'