RFR: 7921: Create 8.3 release notes

[Marcus Hirt]

The fixed issue count my get higher in the end, but I think that's ok.

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Commit messages:
 - 7921: Create 8.3 release notes

Changes: https://git.openjdk.org/jmc/pull/437/files
 Webrev: https://webrevs.openjdk.org/?repo=jmc&pr=437&range=00
  Issue: https://bugs.openjdk.org/browse/JMC-7921
  Stats: 179 lines in 18 files changed: 176 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jmc/pull/437.diff
  Fetch: git fetch https://git.openjdk.org/jmc pull/437/head:pull/437

PR: https://git.openjdk.org/jmc/pull/437