Add a warning about removed courier:batchSearches to 8.0 upgrade assistance #122663
Description
We removed courier:batchSearches that was deprecated for a while in #115681
I think it is worth adding a warning to upgrade assistance so that clusters that were for some reason using the setting a likely to be aware of the change
something similar to: https://github.com/elastic/kibana/blob/7cc56cee5caf708883e93d17ac784fa946b986f5/x-pack/plugins/reporting/server/deprecations/reporting_role.ts