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/omniauth_callbacks_controller_spec.rb

@@ -351,9 +351,9 @@ RSpec.describe Users::OmniauthCallbacksController do
 
       it "should update name/username/email when sso_overrides is enabled" do
         SiteSetting.email_editable = false
-        SiteSetting.sso_overrides_email = true
-        SiteSetting.sso_overrides_name = true
-        SiteSetting.sso_overrides_username = true
+        SiteSetting.auth_overrides_email = true
+        SiteSetting.auth_overrides_name = true
+        SiteSetting.auth_overrides_username = true
 
         UserAssociatedAccount.create!(provider_name: "google_oauth2", user_id: user.id, provider_uid: '123545')
 
@@ -371,7 +371,7 @@ RSpec.describe Users::OmniauthCallbacksController do
 
       it "will not update email if not verified" do
         SiteSetting.email_editable = false
-        SiteSetting.sso_overrides_email = true
+        SiteSetting.auth_overrides_email = true
 
         OmniAuth.config.mock_auth[:google_oauth2][:extra][:raw_info][:email_verified] = false
 
@@ -387,9 +387,9 @@ RSpec.describe Users::OmniauthCallbacksController do
         expect(user.email).to eq('email@example.com')
       end
 
-      it "shows error when sso_overrides_email causes a validation error" do
+      it "shows error when auth_overrides_email causes a validation error" do
         SiteSetting.email_editable = false
-        SiteSetting.sso_overrides_email = true
+        SiteSetting.auth_overrides_email = true
 
         UserAssociatedAccount.create!(provider_name: "google_oauth2", user_id: user.id, provider_uid: '123545')
 
@@ -454,12 +454,12 @@ RSpec.describe Users::OmniauthCallbacksController do
 
       context 'when sso_payload cookie exist' do
         before do
-          SiteSetting.enable_sso_provider = true
-          SiteSetting.sso_secret = "topsecret"
+          SiteSetting.enable_discourse_connect_provider = true
+          SiteSetting.discourse_connect_secret = "topsecret"
 
           @sso = SingleSignOn.new
           @sso.nonce = "mynonce"
-          @sso.sso_secret = SiteSetting.sso_secret
+          @sso.sso_secret = SiteSetting.discourse_connect_secret
           @sso.return_sso_url = "http://somewhere.over.rainbow/sso"
           cookies[:sso_payload] = @sso.payload