Update best practices doc to mention Constructor Property Promotion

[francislavoie]

I think now that v7 requires PHP 8.0+, the https://github.com/PHP-DI/PHP-DI/blob/master/doc/best-practices.md doc should be updated to cover recommending Constructor Property Promotion.

For example, it mentions "This is painful when you have more than 5 dependencies, and your constructor is 15 lines of boilerplate code". This is no longer true using CPP because it keeps it only 5 lines long.

Also, the services example can be simplified by using CPP for its constructor.

[mnapoli]

That's a very good point!