Switch to graphql-transport-ws subprotocol

[MetRonnie]

Closes #690

Implements the graphql-transport-ws subprotocol as defined in https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md

Sibling: cylc/cylc-ui#2548

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).
• No dependency changes
• Tests are included (or explain why tests are not needed).
• Changelog entry included if this is a change that can affect users
• Cylc-Doc pull request opened if required at cylc/cylc-doc/pull/XXXX.
• If this is a bug fix, PR should be raised against the relevant ?.?.x branch.