Callback: [Validator, validate] expects validate to be static

Otherwise you'll get:
```
ContextErrorException: Runtime Notice: call_user_func() expects parameter 1 to be a valid callback, non-static method Vendor\Package\Validator::validate() should not be called statically in Symfony/Component/Validator/Constraints/CallbackValidator.php
```
bug #3570 Callback: [Validator, validate] expects validate to be static (nixilla)

This PR was merged into the 2.4 branch.

Discussion
----------

Callback: [Validator, validate] expects validate to be static

Otherwise you'll get:
```
ContextErrorException: Runtime Notice: call_user_func() expects parameter 1 to be a valid callback, non-static method Vendor\Package\Validator::validate() should not be called statically in Symfony/Component/Validator/Constraints/CallbackValidator.php
```

Commits
-------

027bc97 Callback: [Validator, validate] expects validate to be static
Merge branch '2.3' into 2.4

Conflicts:
	quick_tour/the_big_picture.rst
use DebugClassLoader class from Debug component instead of the one from the Class Loader component