Move GraphQL Playground plugin to another package in this repo

[glasser]

In AS3 we kept around a landing page plugin for GraphQL Playground to help with the migration from AS2. However, GraphQL Playground has been retired and continuing to maintain a wrapper around it doesn't seem like the best priority. We should certainly keep the flexible landing page API that we introduced in AS3, but perhaps we should change from "the apollo-server repo has a dependency on our fork of Playground and exports a plugin" to "we do a one-time publish of a separate package providing an AS4-compatible Playground landing page plugin".

[glasser]

@apollo/server-plugin-legacy-graphql-playground ?

[glasser]

Fixed on version-4 in #6109. Decided to leave 'legacy' out of the package name.

[trevor-scheer]

Revisit:

• Update plugin README.
• Don't support it.
• Mention it in migration guide, but nowhere else.
• Publish once only.

[trevor-scheer]

Closed via #6925