docs: remove unnecessary references to old PG versions

doc/src/sgml/datatype.sgml

doc/src/sgml/ddl.sgml

doc/src/sgml/extend.sgml

doc/src/sgml/func.sgml

doc/src/sgml/libpq.sgml

doc/src/sgml/pgrowlocks.sgml

doc/src/sgml/plpgsql.sgml

doc/src/sgml/plpython.sgml

doc/src/sgml/ref/create_cast.sgml

doc/src/sgml/ref/create_table_as.sgml

doc/src/sgml/ref/pg_config-ref.sgml

doc/src/sgml/ref/reindex.sgml

doc/src/sgml/ref/select_into.sgml

doc/src/sgml/rules.sgml

doc/src/sgml/storage.sgml

doc/src/sgml/xfunc.sgml