RFR: 1102: Document minimum required JDK version 16 for Skara [v3]

[Erik Joelsson]

> Change documentation to reflect that Skara now requires JDK 16 to build.

Erik Joelsson has updated the pull request incrementally with one additional commit since the last revision:

  Force 'release 16' for all java compilations

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Changes:
  - all: https://git.openjdk.java.net/skara/pull/1195/files
  - new: https://git.openjdk.java.net/skara/pull/1195/files/35fa6b8c..373473af

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=skara&pr=1195&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=skara&pr=1195&range=01-02

  Stats: 4 lines in 1 file changed: 4 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/1195.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/1195/head:pull/1195

PR: https://git.openjdk.java.net/skara/pull/1195