RFR: 8248984: Bump minimum boot jdk to JDK 15

[David Holmes]

On Wed, 23 Sep 2020 22:32:37 GMT, Mikael Vidstedt <mikael 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.

Marked as reviewed by dholmes (Reviewer).

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