Postgresql default to no backslash escaping

Changed the default backslash escape value in the PostgreSQL dialect to
``False`` to align it with the default value of
``standard_conforming_strings=on``. This change should not affect most users
since the value is set at driver initialization on first connect.

Fixes: #13268
Change-Id: I9b7986f1ee466fab3cab88e3f6117e313e3376cd