[Validator] Remove property and method targets from the optional and required constraints.

At the moment both constraints can only be defined on other annotations (specifically, the Collection annotation). Defining the required or optional annotation directly on a field or method throws a ClassNotFoundException, since the constraint validator factory tries to load the validator (which does not exist).
Merge branch '2.3' into 2.4

* 2.3:
  [Validator] Remove property and method targets from the optional and required constraints.
  [DomCrawler] Fixed charset detection in html5 meta charset tag