support the graphql-ws subprotocol

[oliver-sanders]

Sibling: https://github.com/cylc/cylc-ui/pull/1429

• The protocol for message exchange employed over a websocket connection is referred as the subprotocol.
• We currently use the graphql-ws subprotocol for serving GraphQL subscriptions over websockets.
• The JavaScript library which implements the grahpql-ws subprotocol is no longer maintained.
• This PR adds support for the graphql-transport-ws subprotocol, allowing us to migrate our client away from this unsupported library.
• This requires https://github.com/graphql-python/graphql-ws/pull/65 and a new release of the Python graphql-ws library to work.

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.