PHPStan check fails on PHP 8

[morozov]

Q	A
Type	improvement
BC Break	no

Fixes #4538:

1. Remove the assertion after array_combine() since none of the static analysis tools flag it as unsafe any longer.
2. Suppress OCI8-related errors until the PHP 7 support is dropped (see Being able to check against a range of PHP versions rather then current runtime phpstan/phpstan#4679 (comment)).

Once the PHP 7 support is dropped, we'll need to adjust some OCI8-related assertions. Not worth doing it right now since it would require suppressing more false positives for Psalm.