graphql-core v3 upgrade

[dwsutherland]

partially addresses #333

Is a sibling to:
cylc/cylc-flow#6478
cylc/cylc-ui#2091
(cylc-flow/cylc-uiserver PR needed to be merged at the same time)

Tests will fail until cylc-flow end is in.

Dependencies on:

graphene-tornado==2.6.*
graphql-ws==0.4.4

have been removed and/or rewritten as a Cylc port of active projects.

There were some short in graphene==3.4.3 comings with the use of middleware and our bespoke execution context (for null-stripping fields) in subscriptions. So I had to adopt and augment a graphql-core package:
https://github.com/graphql-python/graphql-core/blob/v3.2.6/src/graphql/execution/subscribe.py
to our own:
cylc/uiserver/graphql/subscribe.py

The HEAD of graphql-core has this remedied, but both that and the a corresponding graphene will need to be released.
Until then cylc-flow has graphene fixed at 3.4.3, as we'll need to update our code with any future release of graphene/graphql-core (hopefully dropping this workaround).

All aspects appear to be functional:

Check List

• I have read CONTRIBUTING.md and added my name as a Code Contributor.
• Contains logically grouped changes (else tidy your branch by rebase).
• Does not contain off-topic changes (use other PRs for other changes).
• Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users