[Validator] Remove support of boolean value for the checkDNS option

[javiereguiluz]

See symfony/symfony#23391.

The way checkDNS works has changed. I put this issue "on hold" because there's a proposal to revert this change. See symfony/symfony#23538

[eXtreme]

I'm bumping this because 4.0 is out and UrlValidator seems not to accept "true" for checkDNS but docs still say it does.

see: https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Validator/Constraints/UrlValidator.php#L76-L91

[javiereguiluz]

Fixed by #9131.