Summary
-------

Change `Executable.getParameterCount` to be declared to be an `abstract` method, matching its semantics.

Problem
-------

While  `Executable.getParameterCount` is conceptually an abstract method, it is not coded as such, which is confusing. Since `Executable` is now `sealed`, only the implementation in `Mehtod` and `Constructor` are relevant.

Solution
--------
Change the method to be declared as `abstract`.

Specification
-------------

    --- a/src/java.base/share/classes/java/lang/reflect/Executable.java
    +++ b/src/java.base/share/classes/java/lang/reflect/Executable.java
    @@ -253,9 +253,7 @@ public abstract sealed class Executable extends AccessibleObject
          * @return The number of formal parameters for the executable this
          * object represents
          */
    -    public int getParameterCount() {
    -        throw new AbstractMethodError();
    -    }
    +    public abstract int getParameterCount();
     
         /**