[TYPO3-ect] Mark own extension obsolete by version

[Kay Strobach]

Hello Frank,

Thanks for your answers,
but it's just possible to register a new key, transfer it to another
user oder drop the extension completely.

I think the option to mark versions as unsecure / unstable and so
obsolete is important also for extension developers to avoid unsecure
installations. Perhaps there will be a solution after the relaunch?

Best regards
Kay