graphql-ws: migrate from subscriptions-transport-ws to graphql-ws

[oliver-sanders]

Sibling: https://github.com/cylc/cylc-uiserver/pull/482

• The subscriptions-transport-ws library is no longer maintained.
• Move to graphql-ws.
• Closes https://github.com/cylc/cylc-ui/issues/1028

This is currently blocked by lack of support for the graphql-transport-ws subprotocol in the Python graphql-ws library.

Check List

• [x] I have read CONTRIBUTING.md and added my name as a Code Contributor.
• [x] Contains logically grouped changes (else tidy your branch by rebase).
• [x] Does not contain off-topic changes (use other PRs for other changes).
• [x] Applied any dependency changes to both setup.cfg (and conda-environment.yml if present).
• [x] Tests are included (or explain why tests are not needed).
• [x] CHANGES.md entry included if this is a change that can affect users
• [x] Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• [x] If this is a bug fix, PR should be raised against the relevant ?.?.x branch.