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/uploads_controller_spec.rb

@@ -127,8 +127,8 @@ describe UploadsController do
         expect(response.status).to eq(422)
       end
 
-      it 'ensures sso_overrides_avatar is not enabled when uploading an avatar' do
-        SiteSetting.sso_overrides_avatar = true
+      it 'ensures discourse_connect_overrides_avatar is not enabled when uploading an avatar' do
+        SiteSetting.discourse_connect_overrides_avatar = true
         post "/uploads.json", params: { file: logo, type: "avatar" }
         expect(response.status).to eq(422)
       end