Missing prototype replacement section in migration guide

[cathrinevaage]

When migrating a vue 2 app, one would likely refer to the migration guide.
Information about how Vue.prototype.$foo has been replaced by app.config.globalProperties.$foo can not be found.
This is a question that appears pretty frequently in the VueLand vue 3 code help channel, and I think it's deserving of its own section in the migration guide.

[skirtles-code]

At the RFC stage it was documented here:

https://github.com/vuejs/rfcs/blob/master/active-rfcs/0009-global-api-change.md

It seems that it didn't make it through to the equivalent page in the migration guide, perhaps because it was a late addition to the RFC.

It definitely needs to be in the migration guide somewhere.