RFR: 7903868: Release jtreg 7.5

[Christian Stein]

Please review this update to the `CHANGELOG.md` file. This change also includes the increase of the version number for a future release.

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Commit messages:
 - 7903868: Release jtreg 7.5

Changes: https://git.openjdk.org/jtreg/pull/231/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=231&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903868
  Stats: 6 lines in 2 files changed: 4 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/231.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/231/head:pull/231

PR: https://git.openjdk.org/jtreg/pull/231