Replace deprecated UTC datetime helpers

[Mirochill]

Fixes #377.

Summary

• replace deprecated datetime.utcnow() / datetime.utcfromtimestamp(0) usage with timezone-aware UTC timestamps
• update support-access timestamp checks and token age calculation to use datetime.now(timezone.utc).timestamp()
• update history/API-key examples, generated docs, and functional test timestamp helpers

Validation

• git diff --check
• rg -n utcnow|utcfromtimestamp|datetime\.UTC balena tests DOCUMENTATION.md README.md
• Not run locally

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