Correct layerset calls

The Board layerset (because BOARD is a BOARD_ITEM) should return the
layers that the board contains instead of just the F.Cu.  This assigns
the common function to map to the board-specific functions

Fixes https://gitlab.com/kicad/code/kicad/-/issues/20317

(cherry picked from commit 96794e94e3)

pcbnew/board.h

@@ -603,6 +603,7 @@ public:
      * @return the enabled layers in bit-mapped form.
      */
     LSET GetEnabledLayers() const;
+    LSET GetLayerSet() const override { return GetEnabledLayers(); }
 
     /**
      * A proxy function that calls the correspondent function in m_BoardSettings.
@@ -610,6 +611,7 @@ public:
      * @param aLayerMask the new bit-mask of enabled layers.
      */
     void SetEnabledLayers( LSET aLayerMask );
+    void SetLayerSet( const LSET& aLayerMask ) override { SetEnabledLayers( aLayerMask ); }
 
     /**
      * A proxy function that calls the correspondent function in m_BoardSettings