RFR: 8248984: Bump minimum boot jdk to JDK 15

Wed Sep 23 22:50:46 UTC 2020

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.

@rwestberg Note that you will need to update your actions patch if this goes in first.

-------------

Marked as reviewed by erikj (Reviewer).

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