Ubuntu ‘sudo apt get update’ 404 Not Found problem

I am using ubuntu 20.04.3 LTS on my system. While I try to update, I couldn’t update-upgrade or package install on my system and got below errors:

Err:14 http://old-releases.ubuntu.com/ubuntu focal Release                                                                        
  404  Not Found [IP: 80]
Err:15 http://ppa.launchpad.net/team-gcc-arm-embedded/ppa/ubuntu focal Release                                                    
  404  Not Found [IP: 80]
Err:16 http://old-releases.ubuntu.com/ubuntu focal-updates Release                                                           
  404  Not Found [IP: 80]
Hit:17 https://linux-clients.seafile.com/seafile-deb/focal stable InRelease                
Err:18 http://old-releases.ubuntu.com/ubuntu focal-backports Release 
  404  Not Found [IP: 80]
Err:19 http://old-releases.ubuntu.com/ubuntu focal-security Release  
  404  Not Found [IP: 80]
Ign:13 https://launchpad.net/~terry.guo/+archive/gcc-arm-embedded focal InRelease
Err:20 https://launchpad.net/~terry.guo/+archive/gcc-arm-embedded focal Release
  404  Not Found [IP: 443]
Reading package lists... Done


  1. First, restore the default focal repositories using these commands:
mkdir ~/solution
cd ~/solution/
wget https://gist.githubusercontent.com/ishad0w/788555191c7037e249a439542c53e170/raw/3822ba49241e6fd851ca1c1cbcc4d7e87382f484/sources.list
sudo rm -r /etc/apt/sources.list
sudo cp -r ~/solution/sources.list /etc/apt/sources.list

2. Remove all the PPAs in your system:

sudo mv /etc/apt/sources.list.d/* ~/solution

3. Update the repositories:

sudo apt update

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