Log Message:
net/mosquitto: Update to 2.0.21

Upstream changes, less bugfixes and non-pkgsrc platforms.

2.0.21 - 2025-03-06
===================

Security:
- Fix leak on malicious SUBSCRIBE by authenticated client.
  Closes eclipse #248.
- Further fix for CVE-2023-28366.

Broker:
- Add `retain_expiry_interval` option to fix expired retained message not
  being removed from memory if they are not subscribed to. Closes #3221.

Apps:
- mosquitto_ctrl dynsec now also allows `-i` to specify a clientid as well as
  `-c`. This matches the documentation which states `-i`. Closes #3219.