7-8 cylc suite/workflow-state compat

[hjoliver]

Via @dwsutherland today:

The DB polling commands don't work in either direction, and they'll need to as sites transition between Cylc 7 and 8.

(I reported that they did work some time ago, but it seems I had only tested the most basic full-state query)

[hjoliver]

8.1.0
and possibly another Cylc 7 release, although I suppose we could document how to patch the last one for those that need it.

[dwsutherland]

Thanks Hilary, I've had a look at both 👍 .. Let me know when you'd like them reviewed

[MetRonnie]

Closed by #5237