pgsql: 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

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/6973b058bc8d32e104bed99c134a4fab4b5dfe13

Modified Files
--------------
src/backend/catalog/system_views.sql | 5 ++---
src/include/catalog/catversion.h | 2 +-
2 files changed, 3 insertions(+), 4 deletions(-)