Root build.gradle needs to be updated to latest best practices. This is a follow-up of #3446

• Some plugins are declared the old legacy way

  buildscript {
      repositories {
          ...
      }
      dependencies {
          classpath "..."
      }
  }
  
  apply plugin: '...' // actually happening in an applied script.

  This should now use the plugins declaration, and can eventually be improved to use convention plugins rather than scripts.

• Scripts are still in the ./gradle folder which is not standard.

• Nexus publication and shipkit can be applied together as a convention plugin.

• Build script declaration don't seem to be picked up by dependabot or renavabot.

Todo

• Makes license a convention plugin #3532
• Makes shipkit / nexusPublish a convention plugin for root #3533