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.

app/controllers/admin/users_controller.rb

@@ -439,12 +439,12 @@ class Admin::UsersController < Admin::AdminController
   end
 
   def sync_sso
-    return render body: nil, status: 404 unless SiteSetting.enable_sso
+    return render body: nil, status: 404 unless SiteSetting.enable_discourse_connect
 
     begin
       sso = DiscourseSingleSignOn.parse("sso=#{params[:sso]}&sig=#{params[:sig]}")
     rescue DiscourseSingleSignOn::ParseError => e
-      return render json: failed_json.merge(message: I18n.t("sso.login_error")), status: 422
+      return render json: failed_json.merge(message: I18n.t("discourse_connect.login_error")), status: 422
     end
 
     begin
@@ -453,7 +453,7 @@ class Admin::UsersController < Admin::AdminController
     rescue ActiveRecord::RecordInvalid => ex
       render json: failed_json.merge(message: ex.message), status: 403
     rescue DiscourseSingleSignOn::BlankExternalId => ex
-      render json: failed_json.merge(message: I18n.t('sso.blank_id_error')), status: 422
+      render json: failed_json.merge(message: I18n.t('discourse_connect.blank_id_error')), status: 422
     end
   end