Fix incorrect Gradle project configuration.

[zhangzhx]

Issue #, if available:

This issue is reported in AWS Toolkit JetBrains, but the root cause is the Gradle template
aws/aws-toolkit-jetbrains#854

Description of changes:

Remove the redundant sourceSets configuration and replace the deprecated methods.

Checklist:

• Write Design Document (Do I need to write a design document?)
• Write unit tests
• Write/update functional tests
• Write/update integration tests
• make pr passes
• Write documentation

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[sanathkr]

Here is my summary of what I understand from the change. Approving based on this:

• dependencies are updated to remove annotations like not-null, lombok etc
• sourceSet path is set wrong which prevents IDEs from inferring the source code location