Backport of firmware-iwlwifi (20210818-1) to fix major Bluetooth issue

["S." <sb56637@gmail.com>]

Hi everyone, the old version of firmware-iwlwifi that Debian 11 ships has a major bug for Bluetooth connectivity on AX20* chipsets:

- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=987116
- https://www.reddit.com/r/debian/comments/x0rfwu/psa_fix_for_ax200_random_bluetooth_headset/
- https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/issues/1155

I installed the 20210818-1 from Unstable (now I see that the same version is in Testing) and it definitely fixes the problem after **power-cycling** the laptop. It would be great if the 20210818-1 firmware version could be offered in Backports.

Thanks a lot.