Emit deprecation warnings for plain text under Session

Emit deprecation warnings for plain text under Session

Deprecation warnings are emitted under "SQLALCHEMY_WARN_20" mode when
passing a plain string to :meth:`_orm.Session.execute`.

It was also considered to have DDL string expressions to include
this as well, however this leaves us with no backwards-compatible
way of handling reflection of elemens, such as an Index() which
reflects "postgresql_where='x > 5'", there's no place for a rule
that will turn those into text() within the reflection process
that would be separate from when the user passes postgresql_where
to the Index.  Not worth it right now.

Fixes: #5754
Change-Id: I8673a79f0e87de0df576b655f39dad0351725ca8