FEATURE: Rename 'Discourse SSO' to DiscourseConnect (#11978)

The 'Discourse SSO' protocol is being rebranded to DiscourseConnect. This should help to reduce confusion when 'SSO' is used in the generic sense.

This commit aims to:
- Rename `sso_` site settings. DiscourseConnect specific ones are prefixed `discourse_connect_`. Generic settings are prefixed `auth_`
- Add (server-side-only) backwards compatibility for the old setting names, with deprecation notices
- Copy `site_settings` database records to the new names
- Rename relevant translation keys
- Update relevant translations

This commit does **not** aim to:
- Rename any Ruby classes or methods. This might be done in a future commit
- Change any URLs. This would break existing integrations
- Make any changes to the protocol. This would break existing integrations
- Change any functionality. Further normalization across DiscourseConnect and other auth methods will be done separately

The risks are:
- There is no backwards compatibility for site settings on the client-side. Accessing auth-related site settings in Javascript is fairly rare, and an error on the client side would not be security-critical.
- If a plugin is monkey-patching parts of the auth process, changes to locale keys could cause broken error messages. This should also be unlikely. The old site setting names remain functional, so security-related overrides will remain working.

A follow-up commit will be made with a post-deploy migration to delete the old `site_settings` rows.

spec/lib/site_settings/validations_spec.rb

@@ -151,10 +151,10 @@ describe SiteSettings::Validations do
       end
 
       context "when SSO is enabled" do
-        let(:error_message) { I18n.t("errors.site_settings.second_factor_cannot_be_enforced_with_sso_enabled") }
+        let(:error_message) { I18n.t("errors.site_settings.second_factor_cannot_be_enforced_with_discourse_connect_enabled") }
         before do
-          SiteSetting.sso_url = "https://www.example.com/sso"
-          SiteSetting.enable_sso = true
+          SiteSetting.discourse_connect_url = "https://www.example.com/sso"
+          SiteSetting.enable_discourse_connect = true
         end
 
         it "should raise an error" do