Support Java 15 for License Compliance

Release notes

Java 15 is now supported for License Compliance based on customer requests. You can see the full list of supported package managers and languages here: https://docs.gitlab.com/ee/user/compliance/license_compliance/#supported-languages-and-package-managers

Problem to solve

Java 15 came out, we know Java is a popular language choice by our users, we should seek to support it.

Please review current support and plan for what changes need to be made for this to be supported and tested

Implementation plan

1. .gitlab/deb.yml: Add CI job for deb stage for Java 15 .gitlab/deb.yml
2. config/files/.tool-versions: change default java version to 15
3. config/projects/java.rb: change to default fetch version to 15
4. config/software/asdf_java.rb: change default version to 15 and add version "15" block
5. Fix failed tests related to changing default version
6. Add integration tests to spec/integration/java/ to test behaviour with Java 15.
7. Bump version of the analyser and add changelog entry.

Links / references