We can't promise CursorResult from session.execute()

We can't promise CursorResult from session.execute()

Fixed typing bug where the :meth:`.Session.execute` method advertised that
it would return a :class:`.CursorResult` if given an insert/update/delete
statement.  This is not the general case as several flavors of ORM
insert/update do not actually yield a :class:`.CursorResult` which cannot
be differentiated at the typing overload level, so the method now yields
:class:`.Result` in all cases.  For those cases where
:class:`.CursorResult` is known to be returned and the ``.rowcount``
attribute is required, please use ``typing.cast()``.

Fixes: #12813
Change-Id: I8a7197100db312b3898c66ceddd6638e68c6bb44