[v5.4.0] Enhanced upgrader that can handle deprecations #9267
Description
We intend to take the opportunity in v5.4.0 to move some deprecated functionality into optional plugins. For example, #8702 moves almost all of the existing palettes into an optional legacy plugin.
In cases like this it would be useful if the upgrader could detect that a wiki is using one of the deprecated plugins and automatically install the relevant plugin.
We might also explore making it easier to perform batch upgrades using Node.js.
See also the discussion in #9265 about using a custom layout for the upgrader which touches on some related areas.