FIX: Hide old PM settings (#18535)

Follow up to e62e93f83a.
It's confusing and unexpected that deprecated settings
unconditionally change the new setting's value via the
`SiteSettingsController`, even if `override` was set to `false`,
a follow-up PR will correct the issue in `SiteSettingsController`.

config/site_settings.yml

@@ -820,6 +820,7 @@ posting:
   enable_personal_messages:
     default: true
     client: true
+    hidden: true
   enable_system_message_replies:
     default: true
   personal_message_enabled_groups:
@@ -1558,6 +1559,7 @@ trust:
   min_trust_to_send_messages:
     default: 1
     enum: "TrustLevelSetting"
+    hidden: true
   min_trust_to_send_email_messages:
     default: "4"
     enum: "TrustLevelAndStaffSetting"