Disallow extended statistics on system columns

Since introduction of extended statistics, we've disallowed references
to system columns. So for example

    CREATE STATISTICS s ON ctid FROM t;

would fail. But with extended statistics on expressions, it was possible
to work around this limitation quite easily

    CREATE STATISTICS s ON (ctid::text) FROM t;

This is an oversight in a4d75c86bf, fixed by adding a simple check.
Backpatch to PostgreSQL 14, where support for extended statistics on
expressions was introduced.

Backpatch-through: 14
Discussion: https://postgr.es/m/20210816013255.GS10479%40telsasoft.com

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