Admin documentation - possible limitations at Redis server after upgrade to Nc26

From a customer ticket, could be applied here:
https://github.com/nextcloud/documentation/blob/master/admin_manual/release_notes/upgrade_to_26.rst
https://github.com/nextcloud/documentation/blob/master/admin_manual/office/troubleshooting.rst

Problem overview

After upgrading from v24 to v26 there could appear some performance issues.

Web server - Debian12
Redis server - Debian12, redis-server v7.0.11
PHP version: 8.2

Common errors:

• /apps/files/","message":"session_start(): Failed to read session data: redis (path: tcp://[instance:port](http://instance:port)?timeout=10) at /var/ www/nextcloud/lib/private/Session/Internal.php#221"
  -Access forbidden. CSRF check failed at registration
• CSRF check not passed at file upload

Performance bottleneck when creating the redis data was suspected:

• During high-load hours (working time) upload a 1GB text-file fails
• At evening upload of 10GB file goes without problems
• It was also not possible to log out correctly in the web browser
• The command "redis-cli monitor" on the redis-server shows many requests for the user for this time.

Solution

A UFW firewall is running on the redis server. The value `net.netfilter.nf_conntrack_max" was only set to 8192, but the number of simultaneous connections ranges between 6,000 and 12,000. Default value, according to documentation should be around 65536 or higher

Review admin_manual/release_notes/upgrade_to_26.rst and admin_manual/office/troubleshooting.rst, starting with the upgrade notes and the reported Redis connection errors. Document the possible performance and session issues after upgrading, including the nf_conntrack_max limitation and the reported setting guidance. Done means the relevant admin documentation clearly describes the symptoms and solution.