Why I can not install php8.1-redis on my remote server ?

On my remote server (ubuntu under Digital Ocean) with php8.1 I have redis-server installed :

# uname -a
Linux  4.15.0-213-generic #224-Ubuntu SMP Mon Jun 19 13:30:12 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

root@server-remote:/var/www/html/project# php -v
PHP 8.1.9 (cli) (built: Aug 15 2022 09:39:35) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.9, Copyright (c) Zend Technologies
with Zend OPcache v8.1.9, Copyright (c), by Zend Technologies

root@server-remote:/var/www/html/project# apt-get install redis-server
Reading package lists... Done
Building dependency tree
Reading state information... Done
redis-server is already the newest version (5:4.0.9-1ubuntu0.2).
redis-server set to manually installed.
0 upgraded, 0 newly installed, 0 to remove and 6 not upgraded.
root@server-remote:/var/www/html/project# sudo systemctl enable redis-server
Synchronizing state of redis-server.service with SysV service script with /lib/systemd/systemd-sysv-install.
Executing: /lib/systemd/systemd-sysv-install enable redis-server

I failed to install php8.1-redis :

root@server-remote:/var/www/html/project# apt-get install  php8.1-redis
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package php8.1-redis
E: Couldn't find any package by glob 'php8.1-redis'
E: Couldn't find any package by regex 'php8.1-redis'

I know Ubuntu 18.04 LTS is not supported now. Is it reason that I can not install php8.1-redis ? How that ca be fixed ?