F𝗿𝗼𝗺 𝗙𝗲𝗯𝗿𝘂𝗮𝗿𝘆 𝟭, Bitrix24 on premise version need php 8.0 or 8.1 and 𝗮𝗹𝗹 𝘂𝘀𝗲𝗿𝘀 𝗺𝘂𝘀𝘁 𝘂𝗽𝗴𝗿𝗮𝗱𝗲 𝗽𝗵𝗽 to get updates from Bitrix. Also, some problem with portal work may arise for old php versions.

𝗪𝗵𝗮𝘁 𝘄𝗲 𝗿𝗲𝗰𝗼𝗺𝗺𝗲𝗻𝗱:

Update php to 8.0 not to 8.1 because of some problems with 8.1;
If you use BitrixVM just update php via BitrixVM menu;
If you still use a CentOS 7 on your server, you'll need to update php manually (ask in the comments and we'll suggest);
Check the custom modules you use for php 8 updates and install them. If they have not one, be ready to turn these modules off, because custom modules are cause of problem during php 8 update in most cases.