wolfMQTT v1.21.0 (03DEC2025)

Release 1.21.0 has been developed according to wolfSSL's development and QA
process (see link below) and successfully passed the quality criteria.
https://www.wolfssl.com/about/wolfssl-software-development-process-quality-assurance

What's Changed

• Add port for NETX use by @JacobBarthelmeh in #427
• Fix heap buffer overflow in MqttDecode_Num with bounds checking by @devin-ai-integration[bot] in #430
• Restore instructions for testing against OQS mosquitto integration. by @anhu in #432
• initial threadx test by @JacobBarthelmeh in #429
• Rename ML-KEM hybrids to match IETF Draft. by @anhu in #435
• Update expired test certs by @lealem47 in #441
• Pin to ESP-IDF v5.5, limit workflow push branches, line endings by @gojimmypi in #444
• Fix: MQTT v5 Property-Packet Protocol Validation and Decode Safety by @kaabia in #440
• Fix: Correct state transition check in MqttClient_Auth by @kaabia in #439
• fix: correct variable in MQTT property decode error checks by @kaabia in #445
• Add fflush to mqtt-sub example by @embhorn in #447
• Update license to GPLv3 by @embhorn in #448