Support latest rxdart version and align Dart SDK version

[Carapacik]

:sparkles: What kind of change does this PR introduce? (Bug fix, feature, docs update...)

Update versions and align sdk

:arrow_heading_down: What is the current behavior?

:new: What is the new behavior (if this is a feature change)?

:boom: Does this PR introduce a breaking change?

:bug: Recommendations for testing

:memo: Links to relevant issues/docs

:thinking: Checklist before submitting

• [x] All projects build
• [x] Follows style guide lines (code style guide)
• [x] Relevant documentation was updated
• [x] Rebased onto current develop

[Carapacik]

@mvanbeusekom @renefloor check this changes

[hoc081098]

Hi maintainers, please publish it 🙏

[renefloor]

@Carapacik I hope you don't mind that I pushed a commit to your branch. I want to set the version of flutter_lints, so everybody has the same warnings. Also fixed 1 warning and some other versioning stuff.