Update DataTableSkeleton's headers prop to match DataTable's headers prop

Found while working to add TS types for DataTableSkeleton: https://github.com/carbon-design-system/carbon/issues/12516

The DataTableSkeleton propType should match the DataTable propType.
Also, the TypeScript types should be updated as well.
As this change will break the API and require a version bump, it was not done as part of the TS types update.

The DataTable propType for headers is:

    headers: PropTypes.arrayOf(
      PropTypes.shape({
        key: PropTypes.string.isRequired,
        header: PropTypes.node.isRequired,
      })
    ).isRequired,

The DataTableSkeleton propType for headers is:

  headers: PropTypes.oneOfType([
    PropTypes.array,
    PropTypes.shape({
      key: PropTypes.string,
    }),
  ]),

DataTableSkeleton's usage of headers:

                {headers ? (
                  <div className="bx--table-header-label">
                    {headers[i]?.header}
                  </div>
                ) : (
                  <span/>
                )}

carbon-design-system / carbon

Update DataTableSkeleton's headers prop to match DataTable's headers prop #13246