RFR: 7904143: Release jtreg 8.2.1

[Christian Stein]

Please review this change to set jtreg's version number to `8.2.1` and update the `CHANGELOG.md` file to include a section about jtreg 8.2.1

URL targets used in the `CHANGELOG.md` file will start to work after the resulting commit was tagged. For example: https://git.openjdk.org/jtreg/compare/jtreg-8.2.1+1...master

Note that this an empty fixup release.

-------------

Commit messages:
 - 7904143: Release jtreg 8.2.1

Changes: https://git.openjdk.org/jtreg/pull/311/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=311&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904143
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/311.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/311/head:pull/311

PR: https://git.openjdk.org/jtreg/pull/311