Set explicit doctrine/lexer version dependency

[zanardigit]

Bolt v5 is not compatible with doctrine/lexer v3 which introduces a breaking change: the token is now an object of class Doctrine\Common\Lexer\Token and not an array anymore. A normal composer update in a Bolt CMS based project may now generate an error: Cannot use object of type Doctrine\Common\Lexer\Token as array. The error goes away if doctrine/lexer v2 is explicitly required.

On a separate PR I will propose an actual fix. This is about setting the dependency in Bolt CMS own composer.json.