RFR: 7903759: Amend changelog to include the removal of `jtdiff`

[Christian Stein]

Please review this documentation-only PR updating the `CHANGELOG.md` file in preparation of the upcoming `jtreg` 7.5 release.

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Commit messages:
 - 7903759: Amend changelog to include the removal of `jtdiff`

Changes: https://git.openjdk.org/jtreg/pull/220/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=220&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903759
  Stats: 24 lines in 1 file changed: 22 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jtreg/pull/220.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/220/head:pull/220

PR: https://git.openjdk.org/jtreg/pull/220