[stable] remove formal 'unmaintained' phase from EM to EOL transition

Hi stable maintainers! On the Xena Virtual PTG there was a discussion about 'Stable Extended Maintenance to End of Life transition' [1] and at some point we touched that the listed 'Unmaintained' phase [2] does not add value in the process (rather some confusion) so that was agreed that it should be removed (line 397). I've seen that the removal was not yet proposed so I created a patch [3] for it now. So this is just a warning for the change. Thanks, Előd [1] Line 342-397 https://etherpad.opendev.org/p/tc-xena-ptg [2] https://docs.openstack.org/project-team-guide/stable-branches.html#maintenan... [3] https://review.opendev.org/c/openstack/project-team-guide/+/810499