Upgrade to Gradle 6

[malaskowski]

Context Upgrade to Gradle 6.

Describe the solution you'd like Upgrade the Gradle version used in build process to the newest 6 version. This task can be divided into phases:

• PoC - just build with Gradle 6 and try to estimate how much effort is to migrate
• Upgrade - actual upgrade, probably each module will be migrated simultaneously, starting with gradle plugins.
• Verification - ensure are CI pipelines are working.

Additional context Consider using Gradle Update for the future updates.

[tomaszmichalak]

@oskarjerzyk please prepare fix for knotx-docker, knotx-gradle-plugins and knotx-starter-kit.