DEV: Move min_trust_level_for_here_mention to group setting (#24263)

c.f. https://meta.discourse.org/t/-/283408

db/migrate/20231107055903_migrate_min_trust_level_for_here_mention_to_group.rb

@@ -0,0 +1,42 @@
+# frozen_string_literal: true
+
+class MigrateMinTrustLevelForHereMentionToGroup < ActiveRecord::Migration[7.0]
+  def up
+    min_trust_level_for_here_mention_raw =
+      DB.query_single(
+        "SELECT value FROM site_settings WHERE name = 'min_trust_level_for_here_mention'",
+      ).first
+
+    # Default for old setting is trust level 2 and is TrustLevelAndStaffSetting, we only need to do anything if it's been changed in the DB.
+    if min_trust_level_for_here_mention_raw.present?
+      # Matches Group::AUTO_GROUPS to the trust levels & special admin/staff cases.
+      here_mention_allowed_groups =
+        case min_trust_level_for_here_mention_raw
+        when "admin"
+          "1"
+        when "staff"
+          "3"
+        when "0"
+          "10"
+        when "1"
+          "11"
+        when "2"
+          "12"
+        when "3"
+          "13"
+        when "4"
+          "14"
+        end
+
+      # Data_type 20 is group_list.
+      DB.exec(<<~SQL, setting: here_mention_allowed_groups)
+        INSERT INTO site_settings(name, value, data_type, created_at, updated_at)
+        VALUES('here_mention_allowed_groups', :setting, 20, NOW(), NOW())
+      SQL
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end