RFR: Build: Update to JDK 15.0.2

[Erik Helin]

On Fri, 29 Jan 2021 13:50:49 GMT, Andrea Cosentino <github.com+5106647+oscerd at openjdk.org> wrote:

>> Hey @robilad, could you kick off the OCA verification for fellow Red Hatter @oscerd?
>
> Thanks @gunnarmorling :-)

Thanks for contributing @oscerd! Would you mind enabling GitHub Actions in your [fork](https://github.com/oscerd/skara/actions) so that the CI tests can be run?

Thanks!
Erik

-------------

PR: https://git.openjdk.java.net/skara/pull/989