Document appliance upgrade proceedure when using a new appliance image

[agrare]

When upgrading to a version that necessitates a new appliance image there are a few extra steps to backup the old configuration and apply it to the new one. The upgrade to EL9 based appliances will require a new appliance image so we should document this procedure.

[Fryguy]

cc @qahmed1998

[miq-bot]

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