Update rxdart dependency to latest version

[lsuhov]

Hello, thank you for creating this library.

We found that we cannot update bloc dependency to ^3.0.0 due to the following error:

Because bloc_test >=3.0.0 depends on rxdart ^0.23.0 and alphabet_list_scroll_view 1.0.5 depends on rxdart ^0.22.3, bloc_test >=3.0.0 is incompatible with alphabet_list_scroll_view 1.0.5.

And because no versions of alphabet_list_scroll_view match >1.0.5 <2.0.0, bloc_test >=3.0.0 is incompatible with alphabet_list_scroll_view ^1.0.5.

[5hanth]

+1