chore: upgrade rxdart to 0.28.0

[arturograu]

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

Upgrades rxdart to 0.28.0 to allow apps that are depending on that version to compile.

:arrow_heading_down: What is the current behavior?

The apps that depend on this package and on rxdart 0.28.0 don't compile.

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

:boom: Does this PR introduce a breaking change?

Yes, it will not allow apps with previous rxdart versions to build.

:memo: Links to relevant issues/docs

Closes #458

: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