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
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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