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.

lib/auth/result.rb

@@ -70,17 +70,17 @@ class Auth::Result
 
   def apply_user_attributes!
     change_made = false
-    if SiteSetting.sso_overrides_username? && username.present? && username != user.username
+    if SiteSetting.auth_overrides_username? && username.present? && username != user.username
       user.username = UserNameSuggester.suggest(username || name || email, user.username)
       change_made = true
     end
 
-    if SiteSetting.sso_overrides_email && email_valid && email.present? && user.email != Email.downcase(email)
+    if SiteSetting.auth_overrides_email && email_valid && email.present? && user.email != Email.downcase(email)
       user.email = email
       change_made = true
     end
 
-    if SiteSetting.sso_overrides_name && name.present? && user.name != name
+    if SiteSetting.auth_overrides_name && name.present? && user.name != name
       user.name = name
       change_made = true
     end
@@ -89,11 +89,11 @@ class Auth::Result
   end
 
   def can_edit_name
-    !SiteSetting.sso_overrides_name
+    !SiteSetting.auth_overrides_name
   end
 
   def can_edit_username
-    !(SiteSetting.sso_overrides_username || omit_username)
+    !(SiteSetting.auth_overrides_username || omit_username)
   end
 
   def to_client_hash