RFR: Build: Update to JDK 15.0.2

[Andrea Cosentino]

On Tue, 2 Feb 2021 10:53:47 GMT, Andrea Cosentino <github.com+5106647+oscerd at openjdk.org> wrote:

>> 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

I don't know if there is something else to do on my side.

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