[storlets] Transition stable/stein and older branches to EOL

Hi all, I recently noticed that our project still has many old stable branches, from Newton (!!). Because I've never seen any interests to backporting changes to very old branches, I'll propose transitioning stable/stein and older stable branches to EOL. If you have any concerns with that transition please let me know in the following gerrit review. https://review.opendev.org/c/openstack/releases/+/804562 Note that Newton and Ocata will be anywary marked as EOL now since these branches are globally marked as EOL, IIUC. Thank you, Takashi