RFR: 7904094: Release jtreg 8.1

[Christian Stein]

Please review this change to update the `CHANGELOG.md` file to include a section about jtreg 8.1

This change also increases the working number to 8.2

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Commit messages:
 - Start with development of jtreg 8.2
 - 7904094: Release jtreg 8.1

Changes: https://git.openjdk.org/jtreg/pull/294/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=294&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904094
  Stats: 15 lines in 2 files changed: 13 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/294.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/294/head:pull/294

PR: https://git.openjdk.org/jtreg/pull/294