FIX: Reset flair group if user is removed from group (#17862)

The flair used to stay set even if the user was removed from the group.

db/migrate/20220811170600_reset_flair_group_id_if_not_group_member.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+class ResetFlairGroupIdIfNotGroupMember < ActiveRecord::Migration[7.0]
+  def change
+    execute <<~SQL
+      UPDATE users
+      SET flair_group_id = NULL
+      WHERE flair_group_id IS NOT NULL AND NOT EXISTS (
+        SELECT 1
+        FROM group_users
+        WHERE group_users.user_id = users.id
+          AND group_users.group_id = users.flair_group_id
+      )
+    SQL
+  end
+end