[Validator] Remove superfluous versionadded

Author: alexandre-daubois

reference/constraints/Length.rst

@@ -138,10 +138,6 @@ the given value's length is not **exactly** equal to this value.
     ``@Assert\Length(20)`` and ``@Assert\Length(exactly=20)`` are equivalent, as
     well as ``#[Assert\Length(20)]`` and ``#[Assert\Length(exactly: 20)]``.
 
-.. versionadded:: 5.2
-
-    The named argument ``exactly`` was introduced in Symfony 5.2.
-
 exactMessage
 ~~~~~~~~~~~~