Remove internal references to NewDefaultServer

[phughes-scwx]

Here is a pass that removes all references to NewDefaultServer, now deprecated, from three main places:

1. tests that used it for fast and simple setup;
2. the gqlgen init template for a basic server, and the examples based on that template;
3. documentation.

I have made a few personal choices, that may or may not fit with your expectations:

• I have removed the WebSocket and MultipartForm transports from the init template, since they introduce security issues to any server that uses them and should be opted-in to.
• I have extended and changed the wording of some documentation, especially for Introspection and Subscriptions.

[coveralls]

coverage: 59.722% (-0.1%) from 59.869%
when pulling 18967b9 on phughes-scwx:deprecate_new_default_server
into 154c19f on 99designs:deprecate_new_default_server.

[StevenACoffman]

Wow, thanks! That must have been a whole bottle of red!