[v3] Drop legacy API keys

[psrok1]

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Your checklist for this pull request

• [x] I've read the contributing guideline.
• [x] I've tested my changes by building and running the project, and testing changed functionality (if applicable)

What is the current behaviour?

See #304 (Legacy API tokens)

What is the new behaviour?

This PR removes support for unmanaged, manually generated API keys.