Thunderbird: update auto config prefs to match updated patch set (refs: #15091).

The new patch set introduces, or modifies the behaviour of, 4 prefs:

* mailnews.auto_config.account_constraints.allow_oauth2 and
  mailnews.auto_config.account_constraints.ssl_only have been renamed; this
  commit updates our prefs accordingly
* mailnews.auto_config.fetchFromISP.ssl_only → brand new, false by default;
  let's enable it to get the main benefit from our Thunderbird patches
* mailnews.auto_config.guess.timeout → brand new, introduced precisely so that
  we can use a higher value, which I'm doing here.