rails: update settings to 6.1 defaults.

Most of the new settings aren't relevant to us. We do have to fix some
tests to work around a Rails bug. `assert_enqueued_email_with` uses the
wrong queue, so we have to specify it explicitly. This is fixed in Rails
HEAD but not yet released.

test/functional/password_resets_controller_test.rb

@@ -15,7 +15,7 @@ class PasswordResetsControllerTest < ActionDispatch::IntegrationTest
         post password_reset_path, params: { user: { name: @user.name } }
 
         assert_redirected_to new_session_path
-        assert_enqueued_email_with UserMailer, :password_reset, args: [@user]
+        assert_enqueued_email_with UserMailer, :password_reset, args: [@user], queue: "default"
       end
 
       should "should fail if the user doesn't have a verified email address" do