doc: security: Update information about CVE-2021-3581

CVE-2021-3581

-------------

Under embargo until 2021/09/04

HCI data not properly checked leads to memory overflow in the Bluetooth stack

In the process of setting SCAN_RSP through the HCI command, the Zephyr

Bluetooth protocol stack did not effectively check the length of the

incoming HCI data. Causes memory overflow, and then the data in the

memory is overwritten, and may even cause arbitrary code execution.

This has been fixed in main for v2.6.0

- `CVE-2021-3581 <http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2021-3581>`_

- `Zephyr project bug tracker GHSA-8q65-5gqf-fmw5

  <https://github.com/zephyrproject-rtos/zephyr/security/advisories/GHSA-8q65-5gqf-fmw5>`_

- `PR 35935 fix for main

  <https://github.com/zephyrproject-rtos/zephyr/pull/35935>`_

- `PR 35984 fix for 2.5

  <https://github.com/zephyrproject-rtos/zephyr/pull/35984>`_

- `PR 35985 fix for 2.4

  <https://github.com/zephyrproject-rtos/zephyr/pull/35985>`_

- `PR 35985 fix for 1.14

  <https://github.com/zephyrproject-rtos/zephyr/pull/35985>`_