PHP 7.4 will go end of life on 28 November 2022

Just my thoughts: with the aggressive pace of PHP development my knee-jerk reaction is "sooner than later."
However, there are some quirks in 7.4 -> 8.x migration that are haunting us, mainly the changes around null-handling in string functions in PHP 8.x that broke much of our legacy code. With 8.0 on maintenance only, I'd say keep CI compatible with 8.x but don't make it a requirement until 8.0 is unsupported and target CI 5.0 to PHP 8.1 language features.