Make Validator::assert and Validator::assertNullable public

Updated static analyzers

Merge branch '2.6' into 3.6

Revert "Test on PHP 8.1 too (vlucas#467)"

This reverts commit 692830c.

Support boolean values in $_ENV and $_SERVER (vlucas#457)

Merge branch '5.0'

Apply fixes from StyleCI (vlucas#451)

Fixed indentation

Update CONTRIBUTING.md

Tweaks