Raise an error when invalid sort keys are provided by a GraphQL client

[kinow]

These changes close #4114

Transferred from Cylc UI Server repository as I realized the change had to be done in the Cylc Flow schema.

Requirements 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).
• Appropriate tests are included (unit and/or functional).
• Appropriate change log entry included.
• No documentation update required.
• No dependency changes.

[kinow]

Useful for clients like TUI, Cylc UI, and GraphiQL.

What happened in my case was that for Cylc UI I wrote a subscription deltas query in GraphiQL, everything worked, and I thought I could simply copy-paste it into my JavaScript code and prepare a pull request. Later I found out a bug where the sort was not actually working as expected.

With this pull requests clients will know much sooner that there is something wrong with their query, and be able to fix it saving some time.

Cheers
Bruno

[hjoliver]

Change log conflict, and:

./tests/unit/network/test_schema.py:27:15: W291 trailing whitespace

[kinow]

Fixed changelog conflict, and extra space. Thanks @hjoliver !

[hjoliver]

👍