FIX: TL0 could not message group with everyone messageable_level (#19525)

The commits e62e93f83a and
d6bd4ad7ee caused a regression
to the behaviour added for
https://meta.discourse.org/t/allow-tl0-to-write-messages-to-staff-group-not-to-other-members-or-non-staff/124335,
which allowed a user to message a group with the messageable_level
set to Everyone even if they were TL0 (or otherwise did not reach
the appropriate trust level). This commit fixes the issue and adjusts
the spec to reflect the real scenario.

c.f. https://meta.discourse.org/t/tl0-cant-message-groups-with-messageable-level-everyone-recession/249205

spec/lib/guardian_spec.rb

@@ -364,9 +364,9 @@ RSpec.describe Guardian do
       end
     end
 
-    it "allows TL0 to message group with messageable_level = everyone" do
+    it "allows TL0 to message group with messageable_level = everyone regardless of personal_message_enabled_groups" do
       group.update!(messageable_level: Group::ALIAS_LEVELS[:everyone])
-      SiteSetting.personal_message_enabled_groups = Group::AUTO_GROUPS[:trust_level_0]
+      SiteSetting.personal_message_enabled_groups = Group::AUTO_GROUPS[:trust_level_1]
       expect(Guardian.new(trust_level_0).can_send_private_message?(group)).to eq(true)
       expect(Guardian.new(user).can_send_private_message?(group)).to eq(true)
     end