net/mosquitto: Update to 2.0.20

Changes since 2.0.18:

• Fix mismatched subscribe/unsubscribe with normal/shared topics.
• Fix crash on bridge using remapped topic being sent a crafted packet.
• Fix assert failure when loading a persistence file that contains subscriptions with no client id.
• Fix local bridges being incorrectly expired when persistent_client_expiration is in use.
• Fix use of CLOCK_BOOTTIME for getting time.
• Fix mismatched subscribe/unsubscribe with normal/shared topics.
• Fix crash on bridge using remapped topic being sent a crafted packet.
• Fix some error codes being converted to string as "unknown".
• Clear SSL error state to avoid spurious error reporting.
• Fix "payload format invalid" not being allowed as a PUBREC reason code.
• Don't allow SUBACK with missing reason codes.
• Fix QoS 1 / QoS 2 publish incorrectly returning "no subscribers".
• Don't allow invalid response topic values.
• Fix some strict protocol compliance issues.

PR: 283395
Reported by: ari@stonepile.fi
Approved by: joe@thrallingpenguin.com (maintainer timeout, 5 weeks)