Use new ophyd_async soft signal helpers

[DominicOram]

Following bluesky/ophyd-async#217 we should be able to replace some of our helper functions.

Acceptance Criteria

• dodal helper functions are replaced with the equivalent ophyd_async funcs

[DominicOram]

I think this is blocked on some discussion of:

• Writing to soft signals fails in sim bluesky/ophyd-async#267
• Initial values on soft signals not carried through to sim bluesky/ophyd-async#266