Can not upgrade from drupal 8.9.20 to drupal 9

My project use lightning 4 distribution before, but since lightning is not supported any more, so I uninstalled it.
Now I want to upgrade drupal core to 9, I followed instruction :
https://www.drupal.org/docs/upgrading-drupal/how-to-prepare-your-drupal-7-or-8-site-for-drupal-9/upgrading-a-drupal-8-site
and:

City of Quesnel Tourism - Website

Screen shot of website homepage.

The City of Quesnel (area population approx. 23,000) is located in the central interior of British Columbia and is home to beautiful mountain biking and hiking trails, delicious restaurants, lakes, a ski hill and much more! 

Sectors: 
Travel and Hospitality

CI&T Takes Its Website Global with Drupal

CI&T's Global Website mockup

CI&T is a digital specialist that delivers business impact with strategy, design and engineering services.

Sectors: 
Technology

How to save user's session in Redis using Drupal 8 or 9?

Actually, I have a site with vanilla Drupal 8 with latest version. And I have installed Redis in Ubuntu server and also integrated Redis with my Drupal 8 site successfully.

And Now I am able to store all cache in Redis not in default database cache table, after adding below lines in my settings.php file.

YMCA of the North

The YMCA of the North website generates over a million unique visitors per month. With that much traffic, any website improvement project becomes a major undertaking. YMCA of the North contracted Five Jars to do critical development and improve the user experience throughout its website in order to avoid any disruptions to its online services as well as increase brand awareness.

Sectors: 
Non-profit

Drupal based on Symphony Framework -> Lifetime

Building commercial websites is getting worse since Drupal starts to use Symphony Framework.
Why?
f.e.:
Drupal 7 was released 5th January 2011, Commerce 7.0 war available 23th August 2011.
Drupal 7 will reach EOL November 2022
With D7 Vendor Extended Support (D7ES) maybe the end will be 2025
A commercial website was usable for 10 years. Maybe longer with D7ES

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