This PR removes the PaginatedDataTable widget's accentColor dependency per #56918.

The reference in question was used to set the '# items selected' text color when selection is supported.

Breaking change

This is a breaking change, although most apps will not notice the change from theme.accentColor to theme.colorScheme.secondary, because they are typically the same color. For apps that do need a
specific color for the item selection text, you can wrap your PaginatedDataTable with something like:

final ThemeData theme = Theme.of(context);
...
Theme(
  data: theme.copyWith(colorScheme: theme.colorScheme.copyWith(secondary: Colors.red)),
  child: PaginatedDataTable(
    ...
  ),
),

Which will force the selected items header text color red.

A new test was added to verify the selected text header used the secondary color.

This PR was tested against internal Google apps in cl/360773810