Remove everything related to old crux

[dekkers]

Changes

• All code related to crux and non-multinode XTDB has been removed and Octopoes will only support XTDB multinode.
• The code has been updated to use xt/id instead of crux.db/id everywhere (as far as I can see crux.db/id is still supported by XTDB as backwards compatibility so that's why it was still working without problems in the few hardcoded places).
• The option to run robotframework tests with crux has been deleted.
• The robotframework tests have been rewritten to use multinode instead of standalone XTDB. I've also refactored it a bit to get rid of duplication in the test setup/teardown.
• octopoes/docs/source/readme.md was an outdated duplicate of docs/source/developer_documentation/octopoes.md and has been removed.
• All references to crux in the documentation and comments have been updated to XTDB and broken links to opencrux.com have been fixed.
• Celery in the tests doesn't need to use celery beat so I disabled it, I hope this will contribute to making the robot tests more deterministic.

Issue link

Closes #2335

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[Donnype]

Great cleanup. If the tests say aye I say yay