RFR: 1102: Document minimum required JDK version 16 for Skara

Wed Jul 7 17:58:27 UTC 2021

On Wed, 7 Jul 2021 16:01:38 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

> Change documentation to reflect that Skara now requires JDK 16 to build.

You might also want to change the example of `openjdk-15.0.1_osx-x64_bin/jdk-15.0.1.jdk` to `openjdk-16.0.1_osx-x64_bin/jdk-16.0.1.jdk`, but otherwise looks good.

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Marked as reviewed by kcr (Reviewer).

PR: https://git.openjdk.java.net/skara/pull/1195