Further fix privileges on pg_statistic_ext[_data].

Further fix privileges on pg_statistic_ext[_data].

We don't need to restrict column privileges on pg_statistic_ext;
all of that data is OK to read publicly.  What we *do* need to do,
which was overlooked by 6cbfb784c, is revoke public read access on
pg_statistic_ext_data; otherwise we still have the same security
hole we started with.

Catversion bump to ensure that installations calling themselves
beta2 will have this fix.

Diagnosis/correction by Dean Rasheed and Tomas Vondra, but I'm
going to go ahead and push this fix ASAP so we get more buildfarm
cycles on it.

Discussion: https://postgr.es/m/8833.1560647898@sss.pgh.pa.us