change(mqtt): Update submodule to 6af4446a

git log --oneline cac1552e..6af4446a

Detailed description of the changes:
* Multiple event data
  - See merge request espressif/esp-mqtt!240
  - fix: Multiple event data (espressif/esp-mqtt@9c76b70)
* Updated the idf version check for ECDSA support
  - See merge request espressif/esp-mqtt!239
  - fix(mqtt): Updated the idf version check for ECDSA support (espressif/esp-mqtt@7d9a384)
* Added the support for the ECDSA signing
  - See merge request espressif/esp-mqtt!238
  - feat(mqtt): Add support for ECDSA signing (espressif/esp-mqtt@b35a691)
* fix: pubrel message resending when pubcomp not received
  - See merge request espressif/esp-mqtt!236
  - See commit espressif/esp-mqtt@f38a5fc
* Change error message to fix warning.
  - fix: Change error message to fix warning. (espressif/esp-mqtt@c395c2f)
* Small cleanu
  - See merge request espressif/esp-mqtt!231
  - fix: Adds all IDF versions under support (espressif/esp-mqtt@bb85b20)
  - fix: Update badges url to reflect current status (espressif/esp-mqtt@263c167)
* feat(mqtt5): Add parsing of DISCONNECT packet to mqtt5 client
  - mqtt_client: Manage disconnect packet (espressif/esp-mqtt@470cb93)
* feat(mqtt5): Reduce log verbosity for MQTT ACK
  - Reduce log verbosity for MQTT v5 ACKs (espressif/esp-mqtt@6b7b9c3)
* PR: workflows: fix update actions to v4
  - workflows: fix update actions to v4 (espressif/esp-mqtt@06b8923)
* When MQTT_REPORT_DELETED_MESSAGES, delete QOS messages with id 0
  - doc: Clarify deleted message event to cover QoS=0 case (espressif/esp-mqtt@ea036a5)
  - See commit espressif/esp-mqtt@b82c8f6
* fix: Race condition when using destroy
  - See merge request espressif/esp-mqtt!223
  - See commit espressif/esp-mqtt@44292cc
* feat: Add TCP keepalive configuration
  - See commit espressif/esp-mqtt@7c3227a
* feat: Include message topic in all chunks
  - feat: Include message topic in all data events for big messages. (espressif/esp-mqtt@82017e9)
* fix: set TCP transport every time when setting the config
  - See merge request espressif/esp-mqtt!218
  - See commit espressif/esp-mqtt@c3c4cb9
* feat: Allow users to get the transport in use
  - See merge request espressif/esp-mqtt!216
  - See commit espressif/esp-mqtt@97dc85a
* clean: Remove expired deletion from publish
  - See merge request espressif/esp-mqtt!217
  - See commit espressif/esp-mqtt@2285726

Author: euripedesrocha