How to fix `server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt` error when you `git fetch` whatever

Oct 1, 2021. LetsEncrypt's DST Root CA X3 Root Certificate is expired.

As a result, if your client(linux server) is not in the compatible list above, there will be an error similar to this:

server certificate verification failed. CAfile: /etc/ssl/certs/ca-certificates.crt

Are you using Ubuntu 16.04 but not updating it on time?
Then your Ubuntu server couldn't be compatible with ISRG Root X1.
Don't worry, I can help you!

1. create or edit /etc/apt/apt.conf.d/80ssl-exceptions and put this to avoid cert. error during updating.

Acquire::https::Verify-Peer "false"; 
Acquire::https::Verify-Host "false";

2. run this commands

sudo apt-get update
sudo apt-get upgrade

3. don't forget to delete /etc/apt/apt.conf.d/80ssl-exceptions if you created it in step no.1 instead of modifying it.

rm -rf /etc/apt/apt.conf.d/80ssl-exceptions

This is the solution for Ubuntu. I believe that solutions on other platforms will  be shared by experts in each field soon.

Let's save the world.
Good luck!