Use GrahpiQL with Explorer extension instead of vanilla version

[armaniehs]

I started my first spring boot project with DGS this week after working with .NET Core for many years.

One of the first things I noticed is that DGS is shipped with the vanilla version of GraphiQL instead of the extended version with Explorer (https://github.com/OneGraph/graphiql-explorer), which btw is the version used in ASP.NET Core.

The changes are minor so I will provide a PR for it.

[armaniehs]

PR #1388