RFR: 8248984: Bump minimum boot jdk to JDK 15

[Mikael Vidstedt]

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.

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Commit messages:
 - 8248984: Bump minimum boot jdk to JDK 15

Changes: https://git.openjdk.java.net/jdk/pull/326/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=326&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8248984
  Stats: 18 lines in 2 files changed: 0 ins; 13 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/326.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/326/head:pull/326

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