Fix function and view for autovacuum activity

What does this MR do and why?

Fix a view that reads from a set-returning function in !85103 (merged). This is breaking on PG13, which seems to be because it has an additional column in the pg_stat_activity, and the explicit column list in db/structure.sql is not compatible with the values actually returned from the function.

As a result we're getting an off-by-one on the column read, which is manifesting in ERROR: function regexp_matches(xid, text) (because the column before query in pg_stat_activity is backend_xmin, which is an xid).

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