RFR: Build: Update to JDK 15.0.2

[Andrea Cosentino]

On Tue, 2 Feb 2021 10:52:40 GMT, Erik Helin <ehelin at openjdk.org> wrote:

>> 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

@edvbld Done

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