Upgrade GraphQL example to Knot.x 2.2.1

[tomaszmichalak]

Is your feature request related to a problem? Please describe.
Upgrade the GraphQL example (/api-gateway/graphql-api) to the Knot.x 2.2.1.

Describe the solution you'd like
The GraphQL example the following changes:

• upgrade Gradle to 6.X
• configure the new Knot.x Gradle Plugins
• reimplement Actions & Fetchers
• implement unit tests

Additional context
Please upgrade also the tutorial.

[tomaszmichalak]

The code is updated, @Voycawojka can you please update the tutorial?