feat: raise a warning if a GSS connection is obtained using the libpq default

feat: raise a warning if a GSS connection is obtained using the libpq default

Suggest the user to specify a 'gssencmode' setting explicitly so that,
in the future, we will be able to change the setting default to 'disable'.

Note: the warning in psycopg 3.3 is a runtime warning; in 3.2.x was a
deprectation warning.

See #1136