intel-ucode sig=0x406e3 from release 3.20201110.0ubuntu0.20.04.2 hangs system in early boot

My issue: system hang with black screen, and can try older kernels which present status "Loading initial ramdisk..." indefinitely.

The symptom, issue, and the workaround are seemingly identical to those reported in bug#1882890, however the issue here presents with a different microcode update, namely 3.20201110.0ubuntu0.20.04.2.

# reference
* https://bugs.launchpad.net/ubuntu/+source/intel-microcode/+bug/1882890
* https://askubuntu.com/questions/1248769/intel-microcode-3-20200609-0ubuntu0-20-04-0-update-crashes-computer-unable-to-b
* microcode releases
 * https://launchpad.net/ubuntu/focal/+source/intel-microcode

# issue/error
* boot normally gives black screen during early boot - stuck
* boot from previous kernels also give stuck screen, but also helpfully prints "Loading initial ramdisk..." ...indefinitely.
* boot from recovery works, can occasionally boot through to operational system from here
* boot from recovery and running rescue commands complained of near-full /boot (96%) which was subsequently solved (removed old kernels) and initramfs rebuilt (update-initramfs -u -k all) - I thought this was the issue initially but problem remained..
*

# temporary boot workaround via kernel boot option
* can only boot successfully using dis_ucode_ldr kernel boot option:
 * Nov 17 20:16:52 yoga710 kernel: Command line: BOOT_IMAGE=/vmlinuz-5.4.0-54-generic root=/dev/mapper/kubuntu--vg-root ro quiet splash dis_ucode_ldr

## kernel
* uname -r
5.4.0-54-generic

* issue appeared on 2020-11-15 on kernel 5.4.0-53
* kernel updated today/2020-11-17 to 5.4.0-54 from 5.4.0-53 via dis_ucode_ldr booted system

## microcode
microcode: sig=0x406e3 (see below)

* sudo apt show intel-microcode
Package: intel-microcode
Version: 3.20201110.0ubuntu0.20.04.2
[...]
APT-Manual-Installed: yes
APT-Sources: http://ca.archive.ubuntu.com/ubuntu focal-updates/main amd64 Packages

* dmesg | grep microcode (NOTE: this *after* a boot w boot option dis_ucode_ldr)
[ 0.038777] [Firmware Bug]: TSC_DEADLINE disabled due to Errata; please update microcode to version: 0xb2 (or later)
[ 0.125707] SRBDS: Vulnerable: No microcode
[ 0.125708] MDS: Vulnerable: Clear CPU buffers attempted, no microcode

* dmesg | grep microcode (NOTE: this after a boot w microcode downgraded to 3.20191115.1ubuntu3)
[ 0.000000] microcode: microcode updated early to revision 0xd6, date = 2019-10-03
[ 0.131196] SRBDS: Vulnerable: No microcode
[ 0.925593] microcode: sig=0x406e3, pf=0x80, revision=0xd6
[ 0.925717] microcode: Microcode Update Driver: v2.2.

* microcode update date via /var/log/apt/history.log:
Start-Date: 2020-11-12 13:15:29
Commandline: packagekit role='update-packages'
Requested-By: **** (1000)
Install: linux-image-5.4.0-53-generic:amd64 (5.4.0-53.59), linux-headers-5.4.0-53-generic:amd64 (5.4.0-53.59), linux-modules-extra-5.4.0-53-generic:amd64 (5.4.0-53.59), linux-headers-5.4.0-53:amd64 (5.4.0-53.59), linux-modules-5.4.0-53>
Upgrade: libefivar1:amd64 (37-2ubuntu2, 37-2ubuntu2.1), intel-microcode:amd64 (3.20200609.0ubuntu0.20.04.2, 3.20201110.0ubuntu0.20.04.2), linux-headers-generic:amd64 (5.4.0.52.55, 5.4.0.53.56), linux-libc-dev:amd64 (5.4.0-52.57, 5.4.0->
End-Date: 2020-11-12 13:23:05

## processor
* lscpu | grep Model
Model name: Intel(R) Pentium(R) CPU 4405Y @ 1.50GHz

* iucode-tool -S
iucode-tool: system has processor(s) with signature 0x000406e3

## ubuntu
* lsb_release -rd
Description: Ubuntu 20.04.1 LTS
Release: 20.04

## system
* dmidecode
System Information
        Manufacturer: LENOVO
        Product Name: 80TX
        Version: Lenovo YOGA 710-11ISK

BIOS Information
        Vendor: LENOVO
        Version: 0UCN22WW
        Release Date: 09/27/2016
        [...]
  BIOS Revision: 1.22
        Firmware Revision: 1.22

# to try
* check BIOS update status - new BIOS could fix?
 * current: 2016-09-27
 * available: 2018-03-26 0UCN25WW01
  * 3. Modified: 1) Enhancement to address CVE-2017-5715.
 * https://pcsupport.lenovo.com/us/en/products/laptops-and-netbooks/yoga-series/yoga-710-11isk/
 * https://medium.com/@BladeCoder/flash-your-lenovo-ideapad-laptop-bios-from-linux-using-uefi-capsule-updates-a82e455ea29c

# solution attempts
* /boot - made space after a recovery utility complained of near-full (96%) /boot. uninstalled old kernels.

* downgrade and hold microcode
 * sudo apt-get install intel-microcode=3.20191115.1ubuntu3
 * sudo apt-mark hold intel-microcode
 * **ISSUE FIXED WITH THIS WORKAROUND**

* reinstate current microcode to confirm
 * sudo apt install intel-microcode=3.20201110.0ubuntu0.20.04.2
  * "Unpacking intel-microcode (3.20201110.0ubuntu0.20.04.2) over (3.20191115.1ubuntu3)"
  * YES - confirming the problem reappears.