RFR: build: update to JDK 15.0.1

[Robin Westberg]

On Tue, 8 Dec 2020 14:51:18 GMT, Erik Helin <ehelin at openjdk.org> wrote:

> Hi all,
> 
> please review this patch that updates the JDK used for building Skara to 15.0.1.
> 
> Testing:
> - [x] `make images`
> - [x] `make test`
> 
> Thanks,
> Erik

Looks good!

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Marked as reviewed by rwestberg (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/969