Allow PHP 8.6-nightly

[staabm]

DO NOT MERGE

results in https://gist.github.com/staabm/e568ac275bb2671c55904b954f1edc43

this PR is just a vehicle to probe php-src improvements onto the phpstan-src codebase.
current list of candidates is here: https://github.com/ondrejmirtes/php-src/commits/perf/type-check-inline-caches/

[ondrejmirtes]

It's too soon for this. Allowing next PHP version usually comes with a lot of extra work so I usually avoid until feature freeze.

[staabm]

I just need it as a reproducer for others to try. I have no plans in merging it.

without the changes I cannot run phpstan with 8.6 nightly:

➜  phpstan-src git:(pr/5854) php -d memory_limit=599M bin/phpstan -v
Note: Using configuration file /Users/staabm/workspace/phpstan-src/phpstan.neon.dist.
Invalid configuration:
The item 'parameters › phpVersion' expects to be 70100..80599|array|null, 80600 given.