See also: T292847: Make MW 1.37 tests pass on PHP 8.0, T283208: Make MW 1.36 tests pass on PHP 8.0, T274966: Make MW 1.35 tests pass on PHP 8.0; possibly some overlap...
But CI won't run due to older versions of elasticsearch libraries that don't support PHP 8.0 :(
19:42:31 Problem 1 19:42:31 - elasticsearch/elasticsearch v6.5.1 requires php ^7.0 -> your PHP version (8.0.5) does not satisfy that requirement. 19:42:31 - elasticsearch/elasticsearch v6.5.1 requires php ^7.0 -> your PHP version (8.0.5) does not satisfy that requirement. 19:42:31 - elasticsearch/elasticsearch v6.5.1 requires php ^7.0 -> your PHP version (8.0.5) does not satisfy that requirement. 19:42:31 - Installation request for elasticsearch/elasticsearch 6.5.1 -> satisfiable by elasticsearch/elasticsearch[v6.5.1].