Documentation on Omni upgrades (on-prem)

[smira]

Document upgrade procedures:

• when to upgrade
• how to backup Omni before the upgrade (for rollback)
• which version to upgrade to (i.e. what is the latest stable version) (see also #483)
• upgrade path and compatibility (how do I upgrade from vX to vY)
• upgrade notes - e.g. adding /dev/net/tun mount
• what is the minimum supported Omni version on-prem - support policy