Fix grid selection when deleting grid rows or cols

The new selection right/bottom coordinate was off by 1 and could become
invalid.

Closes #24286.

src/generic/gridsel.cpp

@@ -488,7 +488,7 @@ void wxGridSelection::UpdateRows( size_t pos, int numRows )
                         count--;
                     }
                     else
-                        block.SetBottomRow( pos );
+                        block.SetBottomRow( pos - 1 );
                 }
             }
         }
@@ -537,7 +537,7 @@ void wxGridSelection::UpdateCols( size_t pos, int numCols )
                         count--;
                     }
                     else
-                        block.SetRightCol(pos);
+                        block.SetRightCol( pos - 1 );
                 }
             }
         }