[upgrade][neutron] supported release window on rolling upgrade

The neutron team is discussing how many releases we should support in RPC messages [1] (to drop downgrade codes in OVO). This affects rolling upgrade scenarios. Controller nodes are upgrade in FFU way, but we cannot upgrade compute nodes at once. This means controller nodes with N+X release need to talk compute nodes with N release. As of now, the neutron team is thinking to support LTS to LTS upgrade scenarios for major distributions and N->N+4 looks like the longest window.

Rolling upgrade scenarios affect not only neutron but also other projects like nova, so I am sending this mail for broader input.

[1] https://review.opendev.org/#/c/661995/

Thanks, Akihiro Motoki (irc: amotoki)