RFR: Build: Update to JDK 15.0.2

[Gunnar Morling]

On Thu, 28 Jan 2021 18:07:35 GMT, Andrea Cosentino <github.com+5106647+oscerd at openjdk.org> wrote:

>> I believe I'm covered under the blanket Red Hat agreement
>
> Am I supposed to do something now?

Hey @robilad, could you kick off the OCA verification for fellow Red Hatter @oscerd?

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