RFR: 8248984: Bump minimum boot jdk to JDK 15

[Robin Westberg]

On Wed, 23 Sep 2020 22:47:47 GMT, Erik Joelsson <erikj at openjdk.org> wrote:

>> JDK 15 is now GA. The minimum boot JDK version for mainline/JDK 16 should be bumped to this version.
>> 
>> Testing: tier1-5 passed with a slightly earlier version of this change. Re-running tier1 now for good luck.
>
> @rwestberg Note that you will need to update your actions patch if this goes in first.

@erikj79 Thanks for the heads-up, I've updated #284 to use 15 now.

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PR: https://git.openjdk.java.net/jdk/pull/326