RFR: 7904062: Release jtreg 8

[Christian Stein]

Please review this change preparing the release of jtreg 8.

This commit only updates the `CHANGELOG.md` file to include
a selected set of noteworthy changes under the new *8* section.

The `make/build-support/version-numbers` file is already good
to go for the initial 8.x release.

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Commit messages:
 - 7904062: Release jtreg 8

Changes: https://git.openjdk.org/jtreg/pull/273/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=273&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7904062
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/273.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/273/head:pull/273

PR: https://git.openjdk.org/jtreg/pull/273