DEV: Convert min_trust_to_post_links to groups (#25298)

We're changing the implementation of trust levels to use groups. Part of this is to have site settings that reference trust levels use groups instead. It converts the min_trust_to_post_links  site setting to post_links_allowed_groups.

This isn't used by any of our plugins or themes, so very little fallout.

db/migrate/20240117090801_fill_post_links_allowed_groups_based_on_deprecated_settings.rb

@@ -0,0 +1,27 @@
+# frozen_string_literal: true
+
+class FillPostLinksAllowedGroupsBasedOnDeprecatedSettings < ActiveRecord::Migration[7.0]
+  def up
+    configured_trust_level =
+      DB.query_single(
+        "SELECT value FROM site_settings WHERE name = 'min_trust_to_post_links' LIMIT 1",
+      ).first
+
+    # Default for old setting is TL0, we only need to do anything if it's been changed in the DB.
+    if configured_trust_level.present?
+      # Matches Group::AUTO_GROUPS to the trust levels.
+      corresponding_group = "1#{configured_trust_level}"
+
+      # Data_type 20 is group_list.
+      DB.exec(
+        "INSERT INTO site_settings(name, value, data_type, created_at, updated_at)
+        VALUES('post_links_allowed_groups', :setting, '20', NOW(), NOW())",
+        setting: corresponding_group,
+      )
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end