RFR: 1102: Document minimum required JDK version 16 for Skara

[Erik Joelsson]

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

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Commit messages:
 - SKARA-1102

Changes: https://git.openjdk.java.net/skara/pull/1195/files
 Webrev: https://webrevs.openjdk.java.net/?repo=skara&pr=1195&range=00
  Issue: https://bugs.openjdk.java.net/browse/SKARA-1102
  Stats: 6 lines in 1 file changed: 0 ins; 0 del; 6 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