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.

script/benchmarks/site_setting/bench.rb

@@ -14,7 +14,7 @@ SiteSetting.refresh!
 tests = [
   ["current cache", lambda do
     SiteSetting.title
-    SiteSetting.enable_sso
+    SiteSetting.enable_discourse_connect
   end
   ],
   ["change default locale with current cache refreshed", lambda do

script/benchmarks/site_setting/profile.rb

@@ -13,7 +13,7 @@ require File.expand_path('../../../../config/environment', __FILE__)
 
 # warming up
 SiteSetting.title
-SiteSetting.enable_sso
+SiteSetting.enable_discourse_connect
 SiteSetting.default_locale = SiteSetting.default_locale == 'en' ? 'zh_CN' : 'en'
 SiteSetting.title = SecureRandom.hex
 
@@ -22,7 +22,7 @@ profile do
 end
 
 profile do
-  SiteSetting.enable_sso
+  SiteSetting.enable_discourse_connect
 end
 
 profile do