FEATURE: Allow invite only and Discourse connect (#20961)

Invite only and Discourse connect could not be enabled at the same time
because of some legacy reason. This is a follow up commit to ce04db8,
355d51a and 40f6ceb.

spec/lib/validators/enable_sso_validator_spec.rb

@@ -24,24 +24,6 @@ RSpec.describe EnableSsoValidator do
       end
     end
 
-    describe "when invite_only is set" do
-      before do
-        SiteSetting.invite_only = true
-        SiteSetting.discourse_connect_url = "https://example.com/sso"
-      end
-
-      it "allows a false value" do
-        expect(subject.valid_value?("f")).to eq(true)
-      end
-
-      it "doesn't allow true" do
-        expect(subject.valid_value?("t")).to eq(false)
-        expect(subject.error_message).to eq(
-          I18n.t("site_settings.errors.discourse_connect_invite_only"),
-        )
-      end
-    end
-
     describe "when 'sso url' is present" do
       before { SiteSetting.discourse_connect_url = "https://www.example.com/sso" }