RFR: 8257459: Bump minimum boot jdk to JDK 16

[Mikael Vidstedt]

With JDK 16 having gone GA about a month ago it's time to bump the minimum boot JDK version for mainline/JDK 17.

Testing: tier1-5.

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Commit messages:
 - Update GHA configuration
 - 8257459: Bump minimum boot jdk to JDK 16

Changes: https://git.openjdk.java.net/jdk/pull/3555/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3555&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257459
  Stats: 13 lines in 3 files changed: 0 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3555.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3555/head:pull/3555

PR: https://git.openjdk.java.net/jdk/pull/3555