RFR: 7904063: Update version to 8.1

[Christian Stein]

Please review this trivial change starting the development of jtreg 8.1

This PR also fixes comparison links in `CHANGELOG.md` by using the correct build tag: `jtreg-8+2`

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Commit messages:
 - 7904063: Update version to 8.1

Changes: https://git.openjdk.org/jtreg/pull/274/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=274&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904063
  Stats: 3 lines in 2 files changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jtreg/pull/274.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/274/head:pull/274

PR: https://git.openjdk.org/jtreg/pull/274