Proposal: Remove apollo-router-scaffold

[BrynCooke]

In the early days of the Router we had many less features and our users needed to resort to custom plugins to implement functionality that was missing.

We are now in a different world:

• Users are more familiar with Rust.
• We have implemented multiple extension mechanisms for users who don't want to use Rust (co-processors, Rhai)
• We have many more features out of the box.

The apollo-router-scaffold causes us significant issues on CI. Therefore we propose removing the scaffold support.

This does not mean that plugins are removed, but users will have to use our documentation and examples to get started on writing a custom Router plugin.

@shorgi Please can you take a look at what it would take to remove mention of scaffold in the docs and replace with something else?