fix: ensure Gradle 6 support, update test matrix

[kyegupov]

• [x] Ready for review
• [x] Follows CONTRIBUTING rules
• [x] Reviewed by Snyk internal team

What does this PR do?

Updates the test matrix to ensure Gradle 6 support. Also:

• introduces testing for Java 13 and Node 12
• drops non-LTS (unsupported) Java versions
• minimizes the usage of Gradle Wrapper in tests
• suppresses specific tests for unsupported JDK/Gragle versions

Drive-by: clarify the code used to extract the Gradle version.

[snyksec]

:tada: This PR is included in version 3.2.2 :tada:

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot :package::rocket: