RFR: Update to JDK 16 [v2]

[Erik Helin]

> Hi all,
> 
> please review this patch that updates the JDK used for building and running Skara to JDK 16. As part of the update I had to remove `--illegal-access=deny` from Skara's module plugin and I also had use `LogRecord.getLongThreadId` instead of the (since JDK 16) deprecated method `LogRecord.getThreadId`.
> 
> Testing:
> - [x] `make images` locally
> - [x] `make test` locally
> 
> Thanks,
> Erik

Erik Helin has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase.

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1114/files
  - new: https://git.openjdk.java.net/skara/pull/1114/files/49ad26f4..49ad26f4

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1114&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1114&range=00-01

  Stats: 0 lines in 0 files changed: 0 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1114.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1114/head:pull/1114

PR: https://git.openjdk.java.net/skara/pull/1114