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/requests/admin/users_controller_spec.rb

@@ -862,12 +862,12 @@ RSpec.describe Admin::UsersController do
 
     before do
       SiteSetting.email_editable = false
-      SiteSetting.sso_url = "https://www.example.com/sso"
-      SiteSetting.enable_sso = true
-      SiteSetting.sso_overrides_email = true
-      SiteSetting.sso_overrides_name = true
-      SiteSetting.sso_overrides_username = true
-      SiteSetting.sso_secret = sso_secret
+      SiteSetting.discourse_connect_url = "https://www.example.com/sso"
+      SiteSetting.enable_discourse_connect = true
+      SiteSetting.auth_overrides_email = true
+      SiteSetting.auth_overrides_name = true
+      SiteSetting.auth_overrides_username = true
+      SiteSetting.discourse_connect_secret = sso_secret
       sso.sso_secret = sso_secret
     end
 
@@ -924,7 +924,7 @@ RSpec.describe Admin::UsersController do
       correct_payload = Rack::Utils.parse_query(sso.payload)
       post "/admin/users/sync_sso.json", params: correct_payload.merge(sig: "someincorrectsignature")
       expect(response.status).to eq(422)
-      expect(response.parsed_body["message"]).to include(I18n.t('sso.login_error'))
+      expect(response.parsed_body["message"]).to include(I18n.t('discourse_connect.login_error'))
       expect(response.parsed_body["message"]).not_to include(correct_payload["sig"])
     end
 
@@ -935,7 +935,7 @@ RSpec.describe Admin::UsersController do
       sso.external_id = ""
       post "/admin/users/sync_sso.json", params: Rack::Utils.parse_query(sso.payload)
       expect(response.status).to eq(422)
-      expect(response.parsed_body["message"]).to include(I18n.t('sso.blank_id_error'))
+      expect(response.parsed_body["message"]).to include(I18n.t('discourse_connect.blank_id_error'))
     end
   end
 
@@ -1094,8 +1094,8 @@ RSpec.describe Admin::UsersController do
     fab!(:sso_record) { SingleSignOnRecord.create!(user_id: user.id, external_id: '12345', external_email: user.email, last_payload: '') }
 
     it "deletes the record" 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
 
       delete "/admin/users/#{user.id}/sso_record.json"
       expect(response.status).to eq(200)