[spring-server] replace playground with graphiql

[dariuszkuc]

Is your feature request related to a problem? Please describe.
Originally we chose Playground as it was much more feature rich than GraphiQL. Since then project was deprecated in favor of GraphiQL v2 which has full feature parity (and then recently un-deprecated with potential rewrite to reuse GraphiQL internals, still TBD).

Describe the solution you'd like
Update spring-server to bundle GraphiQL instead of Playground.

Describe alternatives you've considered
Since it is just including an extra HTML file we could potentially ship with both.

Additional context
Playground status